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python string to a function call with arguments, without using eval

I have a string stored in a database stands for a class instance creation for example module1.CustomHandler(filename="abc.csv", mode="rb"), where CustomHandler is a class defined in module1.
I would like to evaluate this string to create a class instance for a one time use. Right now I am using something like this
statement = r'module1.CustomHandler(filename="abc.csv", mode="rb")' # actually read from db
exec(f'from parent.module import {statement.split(".")[0]}')
func_or_instance = eval(statement) # this is what I need
Only knowledgable developers can insert such records into database so I am not worried about eval some unwanted codes. But I've read several posts saying eval is unsafe and there is always a better way. Is there a way I can achieve this without using eval?

You might want to take a look at the ast Python module, which stands for abstract syntax trees. It's mainly used when you need to process the grammar of the programming language, work with code in string format, and so much more functions available in the official documentation.
In this case eval() function looks like the best solution, clear and readable, but safe only under certain conditions. For example, if you try to evaluate a code that contains a class not implemented in the code, it will throw an exception. That's the main reason why eval is sometimes unsafe.

Ruby vs Python (the differences between them) [duplicate]

Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.
There is a lot of discussions of Python vs Ruby, and I all find them completely unhelpful, because they all turn around why feature X sucks in language Y, or that claim language Y doesn't have X, although in fact it does. I also know exactly why I prefer Python, but that's also subjective, and wouldn't help anybody choosing, as they might not have the same tastes in development as I do.
It would therefore be interesting to list the differences, objectively. So no "Python's lambdas sucks". Instead explain what Ruby's lambdas can do that Python's can't. No subjectivity. Example code is good!
Don't have several differences in one answer, please. And vote up the ones you know are correct, and down those you know are incorrect (or are subjective). Also, differences in syntax is not interesting. We know Python does with indentation what Ruby does with brackets and ends, and that # is called self in Python.
UPDATE: This is now a community wiki, so we can add the big differences here.
Ruby has a class reference in the class body
In Ruby you have a reference to the class (self) already in the class body. In Python you don't have a reference to the class until after the class construction is finished.
An example:
class Kaka
puts self
end
self in this case is the class, and this code would print out "Kaka". There is no way to print out the class name or in other ways access the class from the class definition body in Python (outside method definitions).
All classes are mutable in Ruby
This lets you develop extensions to core classes. Here's an example of a rails extension:
class String
def starts_with?(other)
head = self[0, other.length]
head == other
end
end
Python (imagine there were no ''.startswith method):
def starts_with(s, prefix):
return s[:len(prefix)] == prefix
You could use it on any sequence (not just strings). In order to use it you should import it explicitly e.g., from some_module import starts_with.
Ruby has Perl-like scripting features
Ruby has first class regexps, $-variables, the awk/perl line by line input loop and other features that make it more suited to writing small shell scripts that munge text files or act as glue code for other programs.
Ruby has first class continuations
Thanks to the callcc statement. In Python you can create continuations by various techniques, but there is no support built in to the language.
Ruby has blocks
With the "do" statement you can create a multi-line anonymous function in Ruby, which will be passed in as an argument into the method in front of do, and called from there. In Python you would instead do this either by passing a method or with generators.
Ruby:
amethod { |here|
many=lines+of+code
goes(here)
}
Python (Ruby blocks correspond to different constructs in Python):
with amethod() as here: # `amethod() is a context manager
many=lines+of+code
goes(here)
Or
for here in amethod(): # `amethod()` is an iterable
many=lines+of+code
goes(here)
Or
def function(here):
many=lines+of+code
goes(here)
amethod(function) # `function` is a callback
Interestingly, the convenience statement in Ruby for calling a block is called "yield", which in Python will create a generator.
Ruby:
def themethod
yield 5
end
themethod do |foo|
puts foo
end
Python:
def themethod():
yield 5
for foo in themethod():
print foo
Although the principles are different, the result is strikingly similar.
Ruby supports functional style (pipe-like) programming more easily
myList.map(&:description).reject(&:empty?).join("\n")
Python:
descriptions = (f.description() for f in mylist)
"\n".join(filter(len, descriptions))
Python has built-in generators (which are used like Ruby blocks, as noted above)
Python has support for generators in the language. In Ruby 1.8 you can use the generator module which uses continuations to create a generator from a block. Or, you could just use a block/proc/lambda! Moreover, in Ruby 1.9 Fibers are, and can be used as, generators, and the Enumerator class is a built-in generator 4
docs.python.org has this generator example:
def reverse(data):
for index in range(len(data)-1, -1, -1):
yield data[index]
Contrast this with the above block examples.
Python has flexible name space handling
In Ruby, when you import a file with require, all the things defined in that file will end up in your global namespace. This causes namespace pollution. The solution to that is Rubys modules. But if you create a namespace with a module, then you have to use that namespace to access the contained classes.
In Python, the file is a module, and you can import its contained names with from themodule import *, thereby polluting the namespace if you want. But you can also import just selected names with from themodule import aname, another or you can simply import themodule and then access the names with themodule.aname. If you want more levels in your namespace you can have packages, which are directories with modules and an __init__.py file.
Python has docstrings
Docstrings are strings that are attached to modules, functions and methods and can be
introspected at runtime. This helps for creating such things as the help command and
automatic documentation.
def frobnicate(bar):
"""frobnicate takes a bar and frobnicates it
>>> bar = Bar()
>>> bar.is_frobnicated()
False
>>> frobnicate(bar)
>>> bar.is_frobnicated()
True
"""
Ruby's equivalent are similar to javadocs, and located above the method instead of within it. They can be retrieved at runtime from the files by using 1.9's Method#source_location example use
Python has multiple inheritance
Ruby does not ("on purpose" -- see Ruby's website, see here how it's done in Ruby). It does reuse the module concept as a type of abstract classes.
Python has list/dict comprehensions
Python:
res = [x*x for x in range(1, 10)]
Ruby:
res = (0..9).map { |x| x * x }
Python:
>>> (x*x for x in range(10))
<generator object <genexpr> at 0xb7c1ccd4>
>>> list(_)
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
Ruby:
p = proc { |x| x * x }
(0..9).map(&p)
Python 2.7+:
>>> {x:str(y*y) for x,y in {1:2, 3:4}.items()}
{1: '4', 3: '16'}
Ruby:
>> Hash[{1=>2, 3=>4}.map{|x,y| [x,(y*y).to_s]}]
=> {1=>"4", 3=>"16"}
Python has decorators
Things similar to decorators can also be created in Ruby, and it can also be argued that they aren't as necessary as in Python.
Syntax differences
Ruby requires "end" or "}" to close all of its scopes, while Python uses white-space only. There have been recent attempts in Ruby to allow for whitespace only indentation http://github.com/michaeledgar/seamless

Ruby has the concepts of blocks, which are essentially syntactic sugar around a section of code; they are a way to create closures and pass them to another method which may or may not use the block. A block can be invoked later on through a yield statement.
For example, a simple definition of an each method on Array might be something like:
class Array
def each
for i in self
yield(i) # If a block has been passed, control will be passed here.
end
end
end
Then you can invoke this like so:
# Add five to each element.
[1, 2, 3, 4].each{ |e| puts e + 5 }
> [6, 7, 8, 9]
Python has anonymous functions/closures/lambdas, but it doesn't quite have blocks since it's missing some of the useful syntactic sugar. However, there's at least one way to get it in an ad-hoc fashion. See, for example, here.

Python Example
Functions are first-class variables in Python. You can declare a function, pass it around as an object, and overwrite it:
def func(): print "hello"
def another_func(f): f()
another_func(func)
def func2(): print "goodbye"
func = func2
This is a fundamental feature of modern scripting languages. JavaScript and Lua do this, too. Ruby doesn't treat functions this way; naming a function calls it.
Of course, there are ways to do these things in Ruby, but they're not first-class operations. For example, you can wrap a function with Proc.new to treat it as a variable--but then it's no longer a function; it's an object with a "call" method.
Ruby's functions aren't first-class objects
Ruby functions aren't first-class objects. Functions must be wrapped in an object to pass them around; the resulting object can't be treated like a function. Functions can't be assigned in a first-class manner; instead, a function in its container object must be called to modify them.
def func; p "Hello" end
def another_func(f); method(f)[] end
another_func(:func) # => "Hello"
def func2; print "Goodbye!"
self.class.send(:define_method, :func, method(:func2))
func # => "Goodbye!"
method(:func).owner # => Object
func # => "Goodbye!"
self.func # => "Goodbye!"

Ultimately all answers are going to be subjective at some level, and the answers posted so far pretty much prove that you can't point to any one feature that isn't doable in the other language in an equally nice (if not similar) way, since both languages are very concise and expressive.
I like Python's syntax. However, you have to dig a bit deeper than syntax to find the true beauty of Ruby. There is zenlike beauty in Ruby's consistency. While no trivial example can possibly explain this completely, I'll try to come up with one here just to explain what I mean.
Reverse the words in this string:
sentence = "backwards is sentence This"
When you think about how you would do it, you'd do the following:
Split the sentence up into words
Reverse the words
Re-join the words back into a string
In Ruby, you'd do this:
sentence.split.reverse.join ' '
Exactly as you think about it, in the same sequence, one method call after another.
In python, it would look more like this:
" ".join(reversed(sentence.split()))
It's not hard to understand, but it doesn't quite have the same flow. The subject (sentence) is buried in the middle. The operations are a mix of functions and object methods. This is a trivial example, but one discovers many different examples when really working with and understanding Ruby, especially on non-trivial tasks.

Python has a "we're all adults here" mentality. Thus, you'll find that Ruby has things like constants while Python doesn't (although Ruby's constants only raise a warning). The Python way of thinking is that if you want to make something constant, you should put the variable names in all caps and not change it.
For example, Ruby:
>> PI = 3.14
=> 3.14
>> PI += 1
(irb):2: warning: already initialized constant PI
=> 4.14
Python:
>>> PI = 3.14
>>> PI += 1
>>> PI
4.1400000000000006

You can import only specific functions from a module in Python. In Ruby, you import the whole list of methods. You could "unimport" them in Ruby, but it's not what it's all about.
EDIT:
let's take this Ruby module :
module Whatever
def method1
end
def method2
end
end
if you include it in your code :
include Whatever
you'll see that both method1 and method2 have been added to your namespace. You can't import only method1. You either import them both or you don't import them at all. In Python you can import only the methods of your choosing. If this would have a name maybe it would be called selective importing?

From Ruby's website:
Similarities
As with Python, in Ruby,...
There’s an interactive prompt (called irb).
You can read docs on the command line (with the ri command instead of pydoc).
There are no special line terminators (except the usual newline).
String literals can span multiple lines like Python’s triple-quoted strings.
Brackets are for lists, and braces are for dicts (which, in Ruby, are called “hashes”).
Arrays work the same (adding them makes one long array, but composing them like this a3 = [ a1, a2 ] gives you an array of arrays).
Objects are strongly and dynamically typed.
Everything is an object, and variables are just references to objects.
Although the keywords are a bit different, exceptions work about the same.
You’ve got embedded doc tools (Ruby’s is called rdoc).
Differences
Unlike Python, in Ruby,...
Strings are mutable.
You can make constants (variables whose value you don’t intend to change).
There are some enforced case-conventions (ex. class names start with a capital letter, variables start with a lowercase letter).
There’s only one kind of list container (an Array), and it’s mutable.
Double-quoted strings allow escape sequences (like \t) and a special “expression substitution” syntax (which allows you to insert the results of Ruby expressions directly into other strings without having to "add " + "strings " + "together"). Single-quoted strings are like Python’s r"raw strings".
There are no “new style” and “old style” classes. Just one kind.
You never directly access attributes. With Ruby, it’s all method calls.
Parentheses for method calls are usually optional.
There’s public, private, and protected to enforce access, instead of Python’s _voluntary_ underscore __convention__.
“mixin’s” are used instead of multiple inheritance.
You can add or modify the methods of built-in classes. Both languages let you open up and modify classes at any point, but Python prevents modification of built-ins — Ruby does not.
You’ve got true and false instead of True and False (and nil instead of None).
When tested for truth, only false and nil evaluate to a false value. Everything else is true (including 0, 0.0, "", and []).
It’s elsif instead of elif.
It’s require instead of import. Otherwise though, usage is the same.
The usual-style comments on the line(s) above things (instead of docstrings below them) are used for generating docs.
There are a number of shortcuts that, although give you more to remember, you quickly learn. They tend to make Ruby fun and very productive.

What Ruby has over Python are its scripting language capabilities. Scripting language in this context meaning to be used for "glue code" in shell scripts and general text manipulation.
These are mostly shared with Perl. First-class built-in regular expressions, $-Variables, useful command line options like Perl (-a, -e) etc.
Together with its terse yet epxressive syntax it is perfect for these kind of tasks.
Python to me is more of a dynamically typed business language that is very easy to learn and has a neat syntax. Not as "cool" as Ruby but neat.
What Python has over Ruby to me is the vast number of bindings for other libs. Bindings to Qt and other GUI libs, many game support libraries and and and. Ruby has much less. While much used bindings e.g. to Databases are of good quality I found niche libs to be better supported in Python even if for the same library there is also a Ruby binding.
So, I'd say both languages have its use and it is the task that defines which one to use. Both are easy enough to learn. I use them side-by-side. Ruby for scripting and Python for stand-alone apps.

I don't think "Ruby has X and Python doesn't, while Python has Y and Ruby doesn't" is the most useful way to look at it. They're quite similar languages, with many shared abilities.
To a large degree, the difference is what the language makes elegant and readable. To use an example you brought up, both do theoretically have lambdas, but Python programmers tend to avoid them, and constructs made using them do not look anywhere near as readable or idiomatic as in Ruby. So in Python, a good programmer will want to take a different route to solving the problem than he would in Ruby, just because it actually is the better way to do it.

I'd like to suggest a variant of the original question, "What does Ruby have that Python doesn't, and vice versa?" which admits the disappointing answer, "Well, what can you do with either Ruby or Python that can't be done in Intercal?" Nothing on that level, because Python and Ruby are both part of the vast royal family sitting on the throne of being Turing approximant.
But what about this:
What can be done gracefully and well in Python that can't be done in Ruby with such beauty and good engineering, or vice versa?
That may be much more interesting than mere feature comparison.

Python has an explicit, builtin syntax for list-comprehenions and generators whereas in Ruby you would use map and code blocks.
Compare
list = [ x*x for x in range(1, 10) ]
to
res = (1..10).map{ |x| x*x }

"Variables that start with a capital letter becomes constants and can't be modified"
Wrong. They can.
You only get a warning if you do.

Somewhat more on the infrastructure side:
Python has much better integration with C++ (via things like Boost.Python, SIP, and Py++) than Ruby, where the options seem to be either write directly against the Ruby interpreter API (which you can do with Python as well, of course, but in both cases doing so is low level, tedious, and error prone) or use SWIG (which, while it works and definitely is great if you want to support many languages, isn't nearly as nice as Boost.Python or SIP if you are specifically looking to bind C++).
Python has a number of web application environments (Django, Pylons/Turbogears, web.py, probably at least half a dozen others), whereas Ruby (effectively) has one: Rails. (Other Ruby web frameworks do exist, but seemingly have a hard time getting much traction against Rails). Is this aspect good or bad? Hard to say, and probably quite subjective; I can easily imagine arguments that the Python situation is better and that the Ruby situation is better.
Culturally, the Python and Ruby communities seem somewhat different, but I can only hint at this as I don't have that much experience interacting with the Ruby community. I'm adding this mostly in the hopes that someone who has a lot of experience with both can amplify (or reject) this statement.

Shamelessly copy/pasted from: Alex Martelli answer on "What's better about Ruby than Python" thread from comp.lang.python mailing list.
Aug 18 2003, 10:50 am Erik Max Francis
wrote:
"Brandon J. Van Every" wrote:
What's better about Ruby than Python? I'm sure there's something.
What is it?
Wouldn't it make much more sense to ask Ruby people this, rather than
Python people?
Might, or might not, depending on
one's purposes -- for example, if
one's purposes include a "sociological
study" of the Python community, then
putting questions to that community is
likely to prove more revealing of
information about it, than putting
them elsewhere:-).
Personally, I gladly took the
opportunity to follow Dave Thomas'
one-day Ruby tutorial at last OSCON.
Below a thin veneer of syntax
differences, I find Ruby and Python
amazingly similar -- if I was
computing the minimum spanning tree
among just about any set of languages,
I'm pretty sure Python and Ruby would
be the first two leaves to coalesce
into an intermediate node:-).
Sure, I do get weary, in Ruby, of
typing the silly "end" at the end of
each block (rather than just
unindenting) -- but then I do get to
avoid typing the equally-silly ':'
which Python requires at the
start of each block, so that's almost a wash:-). Other syntax
differences such as '#foo' versus
'self.foo', or the higher significance
of case in Ruby vs Python, are really
just about as irrelevant to me.
Others no doubt base their choice of
programming languages on just such
issues, and they generate the hottest
debates -- but to me that's just an
example of one of Parkinson's Laws in
action (the amount on debate on an
issue is inversely proportional to the
issue's actual importance).
Edit (by AM 6/19/2010 11:45): this is also known as "painting the
bikeshed" (or, for short,
"bikeshedding") -- the reference is,
again, to Northcote Parkinson, who
gave "debates on what color to paint
the bikeshed" as a typical example of
"hot debates on trivial topics".
(end-of-Edit).
One syntax difference that I do find
important, and in Python's favor --
but other people will no doubt think
just the reverse -- is "how do you
call a function which takes no
parameters". In Python (like in C),
to call a function you always apply
the "call operator" -- trailing
parentheses just after the object
you're calling (inside those trailing
parentheses go the args you're passing
in the call -- if you're passing no
args, then the parentheses are empty).
This leaves the mere mention of
any object, with no operator involved, as meaning just a reference
to the object -- in any context,
without special cases, exceptions,
ad-hoc rules, and the like. In Ruby
(like in Pascal), to call a function
WITH arguments you pass the args
(normally in parentheses, though that
is not invariably the case) -- BUT if
the function takes no args then simply
mentioning the function implicitly
calls it. This may meet the
expectations of many people (at least,
no doubt, those whose only previous
experience of programming was with
Pascal, or other languages with
similar "implicit calling", such as
Visual Basic) -- but to me, it means
the mere mention of an object may
EITHER mean a reference to the object,
OR a call to the object, depending on
the object's type -- and in those
cases where I can't get a reference to
the object by merely mentioning it I
will need to use explicit "give me a
reference to this, DON'T call it!"
operators that aren't needed
otherwise. I feel this impacts the
"first-classness" of functions (or
methods, or other callable objects)
and the possibility of interchanging
objects smoothly. Therefore, to me,
this specific syntax difference is a
serious black mark against Ruby -- but
I do understand why others would thing
otherwise, even though I could hardly
disagree more vehemently with them:-).
Below the syntax, we get into some
important differences in elementary
semantics -- for example, strings in
Ruby are mutable objects (like in
C++), while in Python they are not
mutable (like in Java, or I believe
C#). Again, people who judge
primarily by what they're already
familiar with may think this is a plus
for Ruby (unless they're familiar with
Java or C#, of course:-). Me, I think
immutable strings are an excellent
idea (and I'm not surprised that Java,
independently I think, reinvented that
idea which was already in Python),
though I wouldn't mind having a
"mutable string buffer" type as well
(and ideally one with better
ease-of-use than Java's own "string
buffers"); and I don't give this
judgment because of familiarity --
before studying Java, apart from
functional programming languages where
all data are immutable, all the languages I knew had mutable strings
-- yet when I first saw the immutable-string idea in Java (which I
learned well before I learned Python),
it immediately struck me as excellent,
a very good fit for the
reference-semantics of a higher level
programming language (as opposed to
the value-semantics that fit best with
languages closer to the machine and
farther from applications, such as C)
with strings as a first-class,
built-in (and pretty crucial) data
type.
Ruby does have some advantages in
elementary semantics -- for example,
the removal of Python's "lists vs
tuples" exceedingly subtle
distinction. But mostly the score (as
I keep it, with simplicity a big plus
and subtle, clever distinctions a
notable minus) is against Ruby (e.g.,
having both closed and half-open
intervals, with the notations a..b and
a...b [anybody wants to claim that
it's obvious which is which?-)], is
silly -- IMHO, of course!). Again,
people who consider having a lot of
similar but subtly different things at
the core of a language a PLUS, rather
than a MINUS, will of course count
these "the other way around" from how
I count them:-).
Don't be misled by these comparisons
into thinking the two languages are
very different, mind you. They aren't. But if I'm asked to compare
"capelli d'angelo" to "spaghettini",
after pointing out that these two
kinds of pasta are just about
undistinguishable to anybody and
interchangeable in any dish you might
want to prepare, I would then
inevitably have to move into
microscopic examination of how the
lengths and diameters imperceptibly
differ, how the ends of the strands
are tapered in one case and not in the
other, and so on -- to try and explain
why I, personally, would rather have
capelli d'angelo as the pasta in any
kind of broth, but would prefer
spaghettini as the pastasciutta to go
with suitable sauces for such long
thin pasta forms (olive oil, minced
garlic, minced red peppers, and finely
ground anchovies, for example - but if
you sliced the garlic and peppers
instead of mincing them, then you
should choose the sounder body of
spaghetti rather than the thinner
evanescence of spaghettini, and would
be well advised to forego the achovies
and add instead some fresh spring
basil [or even -- I'm a heretic...! --
light mint...] leaves -- at the very
last moment before serving the dish).
Ooops, sorry, it shows that I'm
traveling abroad and haven't had pasta
for a while, I guess. But the analogy
is still pretty good!-)
So, back to Python and Ruby, we come
to the two biggies (in terms of
language proper -- leaving the
libraries, and other important
ancillaries such as tools and
environments, how to embed/extend each
language, etc, etc, out of it for now
-- they wouldn't apply to all IMPLEMENTATIONS of each language
anyway, e.g., Jython vs Classic Python
being two implementations of the
Python language!):
Ruby's iterators and codeblocks vs Python's iterators and generators;
Ruby's TOTAL, unbridled "dynamicity", including the ability
to "reopen" any existing class,
including all built-in ones, and
change its behavior at run-time -- vs
Python's vast but bounded
dynamicity, which never changes the
behavior of existing built-in
classes and their instances.
Personally, I consider 1 a wash (the
differences are so deep that I could
easily see people hating either
approach and revering the other, but
on MY personal scales the pluses and
minuses just about even up); and 2 a
crucial issue -- one that makes Ruby
much more suitable for "tinkering",
BUT Python equally more suitable for
use in large production applications.
It's funny, in a way, because both
languages are so MUCH more dynamic
than most others, that in the end the
key difference between them from my
POV should hinge on that -- that Ruby
"goes to eleven" in this regard (the
reference here is to "Spinal Tap", of
course). In Ruby, there are no limits
to my creativity -- if I decide that
all string comparisons must become
case-insensitive, I CAN DO THAT!
I.e., I can dynamically alter the
built-in string class so that
a = "Hello World"
b = "hello world"
if a == b
print "equal!\n"
else
print "different!\n"
end WILL print "equal". In python, there is NO way I can do that.
For the purposes of metaprogramming,
implementing experimental frameworks,
and the like, this amazing dynamic
ability of Ruby is extremely
appealing. BUT -- if we're talking
about large applications, developed by
many people and maintained by even
more, including all kinds of libraries
from diverse sources, and needing to
go into production in client sites...
well, I don't WANT a language that is
QUITE so dynamic, thank you very much.
I loathe the very idea of some library
unwittingly breaking other unrelated
ones that rely on those strings being
different -- that's the kind of deep
and deeply hidden "channel", between
pieces of code that LOOK separate and
SHOULD BE separate, that spells
d-e-a-t-h in large-scale programming.
By letting any module affect the
behavior of any other "covertly", the
ability to mutate the semantics of
built-in types is just a BAD idea for
production application programming,
just as it's cool for tinkering.
If I had to use Ruby for such a large
application, I would try to rely on
coding-style restrictions, lots of
tests (to be rerun whenever ANYTHING
changes -- even what should be totally
unrelated...), and the like, to
prohibit use of this language feature.
But NOT having the feature in the
first place is even better, in my
opinion -- just as Python itself would
be an even better language for
application programming if a certain
number of built-ins could be "nailed
down", so I KNEW that, e.g.,
len("ciao") is 4 (rather than having
to worry subliminally about whether
somebody's changed the binding of name
'len' in the builtins module...).
I do hope that eventually Python does
"nail down" its built-ins.
But the problem's minor, since
rebinding built-ins is quite a
deprecated as well as a rare practice
in Python. In Ruby, it strikes me as
major -- just like the too powerful
macro facilities of other languages
(such as, say, Dylan) present similar
risks in my own opinion (I do hope
that Python never gets such a powerful
macro system, no matter the allure of
"letting people define their own
domain-specific little languages
embedded in the language itself" -- it
would, IMHO, impair Python's wonderful
usefulness for application
programming, by presenting an
"attractive nuisance" to the would-be
tinkerer who lurks in every
programmer's heart...).
Alex

Some others from:
http://www.ruby-lang.org/en/documentation/ruby-from-other-languages/to-ruby-from-python/
(If I have misintrepreted anything or any of these have changed on the Ruby side since that page was updated, someone feel free to edit...)
Strings are mutable in Ruby, not in Python (where new strings are created by "changes").
Ruby has some enforced case conventions, Python does not.
Python has both lists and tuples (immutable lists). Ruby has arrays corresponding to Python lists, but no immutable variant of them.
In Python, you can directly access object attributes. In Ruby, it's always via methods.
In Ruby, parentheses for method calls are usually optional, but not in Python.
Ruby has public, private, and protected to enforce access, instead of Python’s convention of using underscores and name mangling.
Python has multiple inheritance. Ruby has "mixins."
And another very relevant link:
http://c2.com/cgi/wiki?PythonVsRuby
Which, in particular, links to another good one by Alex Martelli, who's been also posting a lot of great stuff here on SO:
http://groups.google.com/group/comp.lang.python/msg/028422d707512283

I'm unsure of this, so I add it as an answer first.
Python treats unbound methods as functions
That means you can call a method either like theobject.themethod() or by TheClass.themethod(anobject).
Edit: Although the difference between methods and functions is small in Python, and non-existant in Python 3, it also doesn't exist in Ruby, simply because Ruby doesn't have functions. When you define functions, you are actually defining methods on Object.
But you still can't take the method of one class and call it as a function, you would have to rebind it to the object you want to call on, which is much more obstuse.

I would like to mention Python descriptor API that allows one customize object-to-attribute "communication". It is also noteworthy that, in Python, one is free to implement an alternative protocol via overriding the default given through the default implementation of the __getattribute__ method.
Let me give more details about the aforementioned.
Descriptors are regular classes with __get__, __set__ and/or __delete__ methods.
When interpreter encounters something like anObj.anAttr, the following is performed:
__getattribute__ method of anObj is invoked
__getattribute__ retrieves anAttr object from the class dict
it checks whether abAttr object has __get__, __set__ or __delete__ callable objects
the context (i.e., caller object or class, and value, instead of the latter, if we have setter) is passed to the callable object
the result is returned.
As was mentioned, this is the default behavior. One is free to change the protocol by re-implementing __getattribute__.
This technique is lot more powerful than decorators.

Ruby has builtin continuation support using callcc.
Hence you can implement cool things like the amb-operator

At this stage, Python still has better unicode support

Python has docstrings and ruby doesn't... Or if it doesn't, they are not accessible as easily as in python.
Ps. If im wrong, pretty please, leave an example? I have a workaround that i could monkeypatch into classes quite easily but i'd like to have docstring kinda of a feature in "native way".

Ruby has a line by line loop over input files (the '-n' flag) from the commandline so it can be used like AWK. This Ruby one-liner:
ruby -ne 'END {puts $.}'
will count lines like the AWK one-liner:
awk 'END{print NR}'
Ruby gets feature this through Perl, which took it from AWK as a way of getting sysadmins on board with Perl without having to change the way they do things.

Ruby has sigils and twigils, Python doesn't.
Edit: And one very important thing that I forgot (after all, the previous was just to flame a little bit :-p):
Python has a JIT compiler (Psyco), a sightly lower level language for writing faster code (Pyrex) and the ability to add inline C++ code (Weave).

My python's rusty, so some of these may be in python and i just don't remember/never learned in the first place, but here are the first few that I thought of:
Whitespace
Ruby handles whitespace completely different. For starters, you don't need to indent anything (which means it doesn't matter if you use 4 spaces or 1 tab). It also does smart line continuation, so the following is valid:
def foo(bar,
cow)
Basically, if you end with an operator, it figures out what is going on.
Mixins
Ruby has mixins which can extend instances instead of full classes:
module Humor
def tickle
"hee, hee!"
end
end
a = "Grouchy"
a.extend Humor
a.tickle » "hee, hee!"
Enums
I'm not sure if this is the same as generators, but as of Ruby 1.9 ruby as enums, so
>> enum = (1..4).to_enum
=> #<Enumerator:0x1344a8>
Reference: http://blog.nuclearsquid.com/writings/ruby-1-9-what-s-new-what-s-changed
"Keyword Arguments"
Both of the items listed there are supported in Ruby, although you can't skip default values like that.
You can either go in order
def foo(a, b=2, c=3)
puts "#{a}, #{b}, #{c}"
end
foo(1,3) >> 1, 3, 3
foo(1,c=5) >> 1, 5, 3
c >> 5
Note that c=5 actually assigns the variable c in the calling scope the value 5, and sets the parameter b the value 5.
or you can do it with hashes, which address the second issue
def foo(a, others)
others[:b] = 2 unless others.include?(:b)
others[:c] = 3 unless others.include?(:c)
puts "#{a}, #{others[:b]}, #{others[:c]}"
end
foo(1,:b=>3) >> 1, 3, 3
foo(1,:c=>5) >> 1, 2, 5
Reference: The Pragmatic Progammer's Guide to Ruby

You can have code in the class definition in both Ruby and Python. However, in Ruby you have a reference to the class (self). In Python you don't have a reference to the class, as the class isn't defined yet.
An example:
class Kaka
puts self
end
self in this case is the class, and this code would print out "Kaka". There is no way to print out the class name or in other ways access the class from the class definition body in Python.

Syntax is not a minor thing, it has a direct impact on how we think. It also has a direct effect on the rules we create for the systems we use. As an example we have the order of operations because of the way we write mathematical equations or sentences. The standard notation for mathematics allows people to read it more than one way and arrive at different answers given the same equation. If we had used prefix or postfix notation we would have created rules to distinguish what the numbers to be manipulated were rather than only having rules for the order in which to compute values.
The standard notation makes it plain what numbers we are talking about while making the order in which to compute them ambiguous. Prefix and postfix notation make the order in which to compute plain while making the numbers ambiguous. Python would already have multiline lambdas if it were not for the difficulties caused by the syntactic whitespace. (Proposals do exist for pulling this kind of thing off without necessarily adding explicit block delimiters.)
I find it easier to write conditions where I want something to occur if a condition is false much easier to write with the unless statement in Ruby than the semantically equivalent "if-not" construction in Ruby or other languages for example. If most of the languages that people are using today are equal in power, how can the syntax of each language be considered a trivial thing? After specific features like blocks and inheritance mechanisms etc. syntax is the most important part of a language,hardly a superficial thing.
What is superficial are the aesthetic qualities of beauty that we ascribe to syntax. Aesthetics have nothing to do with how our cognition works, syntax does.

Surprised to see nothing mentioned of ruby's "method missing" mechanism. I'd give examples of the find_by_... methods in Rails, as an example of the power of that language feature. My guess is that something similar could be implemented in Python, but to my knowledge it isn't there natively.

Another difference in lambdas between Python and Ruby is demonstrated by Paul Graham's Accumulator Generator problem. Reprinted here:
Write a function foo that takes a number n and returns a function that takes a number i, and returns n incremented by i.
Note: (a) that's number, not integer, (b) that's incremented by, not plus.
In Ruby, you can do this:
def foo(n)
lambda {|i| n += i }
end
In Python, you'd create an object to hold the state of n:
class foo(object):
def __init__(self, n):
self.n = n
def __call__(self, i):
self.n += i
return self.n
Some folks might prefer the explicit Python approach as being clearer conceptually, even if it's a bit more verbose. You store state like you do for anything else. You just need to wrap your head around the idea of callable objects. But regardless of which approach one prefers aesthetically, it does show one respect in which Ruby lambdas are more powerful constructs than Python's.

python has named optional arguments
def func(a, b=2, c=3):
print a, b, c
>>> func(1)
1 2 3
>>> func(1, c=4)
1 2 4
AFAIK Ruby has only positioned arguments because b=2 in the function declaration is an affectation that always append.

Ruby has embedded documentation:
=begin
You could use rdoc to generate man pages from this documentation
=end

http://c2.com/cgi/wiki?PythonVsRuby
http://c2.com/cgi/wiki?SwitchedFromPythonToRuby
http://c2.com/cgi/wiki?SwitchedFromRubyToPython
http://c2.com/cgi/wiki?UsingPythonDontNeedRuby
http://c2.com/cgi/wiki?UsingRubyDontNeedPython

In Ruby, when you import a file with
require, all the things defined in
that file will end up in your global
namespace.
With Cargo you can "require libraries without cluttering your namespace".
# foo-1.0.0.rb
class Foo
VERSION = "1.0.0"
end
# foo-2.0.0.rb
class Foo
VERSION = "2.0.0"
end
>> Foo1 = import("foo-1.0.0")
>> Foo2 = import("foo-2.0.0")
>> Foo1::VERSION
=> "1.0.0"
>> Foo2::VERSION
=> "2.0.0"

Hot swapping python code (duck type functions?)

I've been thinking about this far too long and haven't gotten any idea, maybe some of you can help.
I have a folder of python scripts, all of which have the same surrounding body (literally, I generated it from a shell script), but have one chunk that's different than all of them. In other words:
Top piece of code (always the same)
Middle piece of code (changes from file to file)
Bottom piece of code (always the same)
And I realized today that this is a bad idea, for example, if I want to change something from the top or bottom sections, I need to write a shell script to do it. (Not that that's hard, it just seems like it's very bad code wise).
So what I want to do, is have one outer python script that is like this:
Top piece of code
Dynamic function that calls the middle piece of code (based on a parameter)
Bottom piece of code
And then every other python file in the folder can simply be the middle piece of code. However, normal module wouldn't work here (unless I'm mistaken), because I would get the code I need to execute from the arguement, which would be a string, and thus I wouldn't know which function to run until runtime.
So I thought up two more solutions:
I could write up a bunch of if statements, one to run each script based on a certain parameter. I rejected this, as it's even worse than the previous design.
I could use:
os.command(sys.argv[0] scriptName.py)
which would run the script, but calling python to call python doesn't seem very elegant to me.
So does anyone have any other ideas? Thank you.

If you know the name of the function as a string and the name of module as a string, then you can do
mod = __import__(module_name)
fn = getattr(mod, fn_name)
fn()

Another possible solution is to have each of your repetitive files import the functionality from the main file
from topAndBottom import top, bottom
top()
# do middle stuff
bottom()

In addition to the several answers already posted, consider the Template Method design pattern: make an abstract class such as
class Base(object):
def top(self): ...
def bottom(self): ...
def middle(self): raise NotImplementedError
def doit(self):
self.top()
self.middle()
self.bottom()
Every pluggable module then makes a class which inherits from this Base and must override middle with the relevant code.
Perhaps not warranted for this simple case (you do still have to import the right module in order to instantiate its class and call doit on it), but still worth keeping in mind (together with its many Pythonic variations, which I have amply explained in many tech talks now available on youtube) for cases where the number or complexity of "pluggable pieces" keeps growing -- Template Method (despite its horrid name;-) is a solid, well-proven and highly scalable pattern [[sometimes a tad too rigid, but that's exactly what I address in those many tech talks -- and that problem doesn't apply to this specific use case]].

However, normal module wouldn't work here (unless I'm mistaken), because I would get the code I need to execute from the arguement, which would be a string, and thus I wouldn't know which function to run until runtime.
It will work just fine - use __import__ builtin or, if you have very complex layout, imp module to import your script. And then you can get the function by module.__dict__[funcname] for example.

Importing a module (as explained in other answers) is definitely the cleaner way to do this, but if for some reason that doesn't work, as long as you're not doing anything too weird you can use exec. It basically runs the content of another file as if it were included in the current file at the point where exec is called. It's the closest thing Python has to a source statement of the kind included in many shells. As a bare minimum, something like this should work:
exec(open(filename).read(None))

How about this?
function do_thing_one():
pass
function do_thing_two():
pass
dispatch = { "one" : do_thing_one,
"two" : do_thing_two,
}
# do something to get your string from the command line (optparse, argv, whatever)
# and put it in variable "mystring"
# do top thing
f = dispatch[mystring]
f()
# do bottom thing

What does Ruby have that Python doesn't, and vice versa?

Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.
There is a lot of discussions of Python vs Ruby, and I all find them completely unhelpful, because they all turn around why feature X sucks in language Y, or that claim language Y doesn't have X, although in fact it does. I also know exactly why I prefer Python, but that's also subjective, and wouldn't help anybody choosing, as they might not have the same tastes in development as I do.
It would therefore be interesting to list the differences, objectively. So no "Python's lambdas sucks". Instead explain what Ruby's lambdas can do that Python's can't. No subjectivity. Example code is good!
Don't have several differences in one answer, please. And vote up the ones you know are correct, and down those you know are incorrect (or are subjective). Also, differences in syntax is not interesting. We know Python does with indentation what Ruby does with brackets and ends, and that # is called self in Python.
UPDATE: This is now a community wiki, so we can add the big differences here.
Ruby has a class reference in the class body
In Ruby you have a reference to the class (self) already in the class body. In Python you don't have a reference to the class until after the class construction is finished.
An example:
class Kaka
puts self
end
self in this case is the class, and this code would print out "Kaka". There is no way to print out the class name or in other ways access the class from the class definition body in Python (outside method definitions).
All classes are mutable in Ruby
This lets you develop extensions to core classes. Here's an example of a rails extension:
class String
def starts_with?(other)
head = self[0, other.length]
head == other
end
end
Python (imagine there were no ''.startswith method):
def starts_with(s, prefix):
return s[:len(prefix)] == prefix
You could use it on any sequence (not just strings). In order to use it you should import it explicitly e.g., from some_module import starts_with.
Ruby has Perl-like scripting features
Ruby has first class regexps, $-variables, the awk/perl line by line input loop and other features that make it more suited to writing small shell scripts that munge text files or act as glue code for other programs.
Ruby has first class continuations
Thanks to the callcc statement. In Python you can create continuations by various techniques, but there is no support built in to the language.
Ruby has blocks
With the "do" statement you can create a multi-line anonymous function in Ruby, which will be passed in as an argument into the method in front of do, and called from there. In Python you would instead do this either by passing a method or with generators.
Ruby:
amethod { |here|
many=lines+of+code
goes(here)
}
Python (Ruby blocks correspond to different constructs in Python):
with amethod() as here: # `amethod() is a context manager
many=lines+of+code
goes(here)
Or
for here in amethod(): # `amethod()` is an iterable
many=lines+of+code
goes(here)
Or
def function(here):
many=lines+of+code
goes(here)
amethod(function) # `function` is a callback
Interestingly, the convenience statement in Ruby for calling a block is called "yield", which in Python will create a generator.
Ruby:
def themethod
yield 5
end
themethod do |foo|
puts foo
end
Python:
def themethod():
yield 5
for foo in themethod():
print foo
Although the principles are different, the result is strikingly similar.
Ruby supports functional style (pipe-like) programming more easily
myList.map(&:description).reject(&:empty?).join("\n")
Python:
descriptions = (f.description() for f in mylist)
"\n".join(filter(len, descriptions))
Python has built-in generators (which are used like Ruby blocks, as noted above)
Python has support for generators in the language. In Ruby 1.8 you can use the generator module which uses continuations to create a generator from a block. Or, you could just use a block/proc/lambda! Moreover, in Ruby 1.9 Fibers are, and can be used as, generators, and the Enumerator class is a built-in generator 4
docs.python.org has this generator example:
def reverse(data):
for index in range(len(data)-1, -1, -1):
yield data[index]
Contrast this with the above block examples.
Python has flexible name space handling
In Ruby, when you import a file with require, all the things defined in that file will end up in your global namespace. This causes namespace pollution. The solution to that is Rubys modules. But if you create a namespace with a module, then you have to use that namespace to access the contained classes.
In Python, the file is a module, and you can import its contained names with from themodule import *, thereby polluting the namespace if you want. But you can also import just selected names with from themodule import aname, another or you can simply import themodule and then access the names with themodule.aname. If you want more levels in your namespace you can have packages, which are directories with modules and an __init__.py file.
Python has docstrings
Docstrings are strings that are attached to modules, functions and methods and can be
introspected at runtime. This helps for creating such things as the help command and
automatic documentation.
def frobnicate(bar):
"""frobnicate takes a bar and frobnicates it
>>> bar = Bar()
>>> bar.is_frobnicated()
False
>>> frobnicate(bar)
>>> bar.is_frobnicated()
True
"""
Ruby's equivalent are similar to javadocs, and located above the method instead of within it. They can be retrieved at runtime from the files by using 1.9's Method#source_location example use
Python has multiple inheritance
Ruby does not ("on purpose" -- see Ruby's website, see here how it's done in Ruby). It does reuse the module concept as a type of abstract classes.
Python has list/dict comprehensions
Python:
res = [x*x for x in range(1, 10)]
Ruby:
res = (0..9).map { |x| x * x }
Python:
>>> (x*x for x in range(10))
<generator object <genexpr> at 0xb7c1ccd4>
>>> list(_)
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
Ruby:
p = proc { |x| x * x }
(0..9).map(&p)
Python 2.7+:
>>> {x:str(y*y) for x,y in {1:2, 3:4}.items()}
{1: '4', 3: '16'}
Ruby:
>> Hash[{1=>2, 3=>4}.map{|x,y| [x,(y*y).to_s]}]
=> {1=>"4", 3=>"16"}
Python has decorators
Things similar to decorators can also be created in Ruby, and it can also be argued that they aren't as necessary as in Python.
Syntax differences
Ruby requires "end" or "}" to close all of its scopes, while Python uses white-space only. There have been recent attempts in Ruby to allow for whitespace only indentation http://github.com/michaeledgar/seamless

Ruby has the concepts of blocks, which are essentially syntactic sugar around a section of code; they are a way to create closures and pass them to another method which may or may not use the block. A block can be invoked later on through a yield statement.
For example, a simple definition of an each method on Array might be something like:
class Array
def each
for i in self
yield(i) # If a block has been passed, control will be passed here.
end
end
end
Then you can invoke this like so:
# Add five to each element.
[1, 2, 3, 4].each{ |e| puts e + 5 }
> [6, 7, 8, 9]
Python has anonymous functions/closures/lambdas, but it doesn't quite have blocks since it's missing some of the useful syntactic sugar. However, there's at least one way to get it in an ad-hoc fashion. See, for example, here.

Python Example
Functions are first-class variables in Python. You can declare a function, pass it around as an object, and overwrite it:
def func(): print "hello"
def another_func(f): f()
another_func(func)
def func2(): print "goodbye"
func = func2
This is a fundamental feature of modern scripting languages. JavaScript and Lua do this, too. Ruby doesn't treat functions this way; naming a function calls it.
Of course, there are ways to do these things in Ruby, but they're not first-class operations. For example, you can wrap a function with Proc.new to treat it as a variable--but then it's no longer a function; it's an object with a "call" method.
Ruby's functions aren't first-class objects
Ruby functions aren't first-class objects. Functions must be wrapped in an object to pass them around; the resulting object can't be treated like a function. Functions can't be assigned in a first-class manner; instead, a function in its container object must be called to modify them.
def func; p "Hello" end
def another_func(f); method(f)[] end
another_func(:func) # => "Hello"
def func2; print "Goodbye!"
self.class.send(:define_method, :func, method(:func2))
func # => "Goodbye!"
method(:func).owner # => Object
func # => "Goodbye!"
self.func # => "Goodbye!"

Ultimately all answers are going to be subjective at some level, and the answers posted so far pretty much prove that you can't point to any one feature that isn't doable in the other language in an equally nice (if not similar) way, since both languages are very concise and expressive.
I like Python's syntax. However, you have to dig a bit deeper than syntax to find the true beauty of Ruby. There is zenlike beauty in Ruby's consistency. While no trivial example can possibly explain this completely, I'll try to come up with one here just to explain what I mean.
Reverse the words in this string:
sentence = "backwards is sentence This"
When you think about how you would do it, you'd do the following:
Split the sentence up into words
Reverse the words
Re-join the words back into a string
In Ruby, you'd do this:
sentence.split.reverse.join ' '
Exactly as you think about it, in the same sequence, one method call after another.
In python, it would look more like this:
" ".join(reversed(sentence.split()))
It's not hard to understand, but it doesn't quite have the same flow. The subject (sentence) is buried in the middle. The operations are a mix of functions and object methods. This is a trivial example, but one discovers many different examples when really working with and understanding Ruby, especially on non-trivial tasks.

Python has a "we're all adults here" mentality. Thus, you'll find that Ruby has things like constants while Python doesn't (although Ruby's constants only raise a warning). The Python way of thinking is that if you want to make something constant, you should put the variable names in all caps and not change it.
For example, Ruby:
>> PI = 3.14
=> 3.14
>> PI += 1
(irb):2: warning: already initialized constant PI
=> 4.14
Python:
>>> PI = 3.14
>>> PI += 1
>>> PI
4.1400000000000006

You can import only specific functions from a module in Python. In Ruby, you import the whole list of methods. You could "unimport" them in Ruby, but it's not what it's all about.
EDIT:
let's take this Ruby module :
module Whatever
def method1
end
def method2
end
end
if you include it in your code :
include Whatever
you'll see that both method1 and method2 have been added to your namespace. You can't import only method1. You either import them both or you don't import them at all. In Python you can import only the methods of your choosing. If this would have a name maybe it would be called selective importing?

From Ruby's website:
Similarities
As with Python, in Ruby,...
There’s an interactive prompt (called irb).
You can read docs on the command line (with the ri command instead of pydoc).
There are no special line terminators (except the usual newline).
String literals can span multiple lines like Python’s triple-quoted strings.
Brackets are for lists, and braces are for dicts (which, in Ruby, are called “hashes”).
Arrays work the same (adding them makes one long array, but composing them like this a3 = [ a1, a2 ] gives you an array of arrays).
Objects are strongly and dynamically typed.
Everything is an object, and variables are just references to objects.
Although the keywords are a bit different, exceptions work about the same.
You’ve got embedded doc tools (Ruby’s is called rdoc).
Differences
Unlike Python, in Ruby,...
Strings are mutable.
You can make constants (variables whose value you don’t intend to change).
There are some enforced case-conventions (ex. class names start with a capital letter, variables start with a lowercase letter).
There’s only one kind of list container (an Array), and it’s mutable.
Double-quoted strings allow escape sequences (like \t) and a special “expression substitution” syntax (which allows you to insert the results of Ruby expressions directly into other strings without having to "add " + "strings " + "together"). Single-quoted strings are like Python’s r"raw strings".
There are no “new style” and “old style” classes. Just one kind.
You never directly access attributes. With Ruby, it’s all method calls.
Parentheses for method calls are usually optional.
There’s public, private, and protected to enforce access, instead of Python’s _voluntary_ underscore __convention__.
“mixin’s” are used instead of multiple inheritance.
You can add or modify the methods of built-in classes. Both languages let you open up and modify classes at any point, but Python prevents modification of built-ins — Ruby does not.
You’ve got true and false instead of True and False (and nil instead of None).
When tested for truth, only false and nil evaluate to a false value. Everything else is true (including 0, 0.0, "", and []).
It’s elsif instead of elif.
It’s require instead of import. Otherwise though, usage is the same.
The usual-style comments on the line(s) above things (instead of docstrings below them) are used for generating docs.
There are a number of shortcuts that, although give you more to remember, you quickly learn. They tend to make Ruby fun and very productive.

What Ruby has over Python are its scripting language capabilities. Scripting language in this context meaning to be used for "glue code" in shell scripts and general text manipulation.
These are mostly shared with Perl. First-class built-in regular expressions, $-Variables, useful command line options like Perl (-a, -e) etc.
Together with its terse yet epxressive syntax it is perfect for these kind of tasks.
Python to me is more of a dynamically typed business language that is very easy to learn and has a neat syntax. Not as "cool" as Ruby but neat.
What Python has over Ruby to me is the vast number of bindings for other libs. Bindings to Qt and other GUI libs, many game support libraries and and and. Ruby has much less. While much used bindings e.g. to Databases are of good quality I found niche libs to be better supported in Python even if for the same library there is also a Ruby binding.
So, I'd say both languages have its use and it is the task that defines which one to use. Both are easy enough to learn. I use them side-by-side. Ruby for scripting and Python for stand-alone apps.

I don't think "Ruby has X and Python doesn't, while Python has Y and Ruby doesn't" is the most useful way to look at it. They're quite similar languages, with many shared abilities.
To a large degree, the difference is what the language makes elegant and readable. To use an example you brought up, both do theoretically have lambdas, but Python programmers tend to avoid them, and constructs made using them do not look anywhere near as readable or idiomatic as in Ruby. So in Python, a good programmer will want to take a different route to solving the problem than he would in Ruby, just because it actually is the better way to do it.

I'd like to suggest a variant of the original question, "What does Ruby have that Python doesn't, and vice versa?" which admits the disappointing answer, "Well, what can you do with either Ruby or Python that can't be done in Intercal?" Nothing on that level, because Python and Ruby are both part of the vast royal family sitting on the throne of being Turing approximant.
But what about this:
What can be done gracefully and well in Python that can't be done in Ruby with such beauty and good engineering, or vice versa?
That may be much more interesting than mere feature comparison.

Python has an explicit, builtin syntax for list-comprehenions and generators whereas in Ruby you would use map and code blocks.
Compare
list = [ x*x for x in range(1, 10) ]
to
res = (1..10).map{ |x| x*x }

"Variables that start with a capital letter becomes constants and can't be modified"
Wrong. They can.
You only get a warning if you do.

Somewhat more on the infrastructure side:
Python has much better integration with C++ (via things like Boost.Python, SIP, and Py++) than Ruby, where the options seem to be either write directly against the Ruby interpreter API (which you can do with Python as well, of course, but in both cases doing so is low level, tedious, and error prone) or use SWIG (which, while it works and definitely is great if you want to support many languages, isn't nearly as nice as Boost.Python or SIP if you are specifically looking to bind C++).
Python has a number of web application environments (Django, Pylons/Turbogears, web.py, probably at least half a dozen others), whereas Ruby (effectively) has one: Rails. (Other Ruby web frameworks do exist, but seemingly have a hard time getting much traction against Rails). Is this aspect good or bad? Hard to say, and probably quite subjective; I can easily imagine arguments that the Python situation is better and that the Ruby situation is better.
Culturally, the Python and Ruby communities seem somewhat different, but I can only hint at this as I don't have that much experience interacting with the Ruby community. I'm adding this mostly in the hopes that someone who has a lot of experience with both can amplify (or reject) this statement.

Shamelessly copy/pasted from: Alex Martelli answer on "What's better about Ruby than Python" thread from comp.lang.python mailing list.
Aug 18 2003, 10:50 am Erik Max Francis
wrote:
"Brandon J. Van Every" wrote:
What's better about Ruby than Python? I'm sure there's something.
What is it?
Wouldn't it make much more sense to ask Ruby people this, rather than
Python people?
Might, or might not, depending on
one's purposes -- for example, if
one's purposes include a "sociological
study" of the Python community, then
putting questions to that community is
likely to prove more revealing of
information about it, than putting
them elsewhere:-).
Personally, I gladly took the
opportunity to follow Dave Thomas'
one-day Ruby tutorial at last OSCON.
Below a thin veneer of syntax
differences, I find Ruby and Python
amazingly similar -- if I was
computing the minimum spanning tree
among just about any set of languages,
I'm pretty sure Python and Ruby would
be the first two leaves to coalesce
into an intermediate node:-).
Sure, I do get weary, in Ruby, of
typing the silly "end" at the end of
each block (rather than just
unindenting) -- but then I do get to
avoid typing the equally-silly ':'
which Python requires at the
start of each block, so that's almost a wash:-). Other syntax
differences such as '#foo' versus
'self.foo', or the higher significance
of case in Ruby vs Python, are really
just about as irrelevant to me.
Others no doubt base their choice of
programming languages on just such
issues, and they generate the hottest
debates -- but to me that's just an
example of one of Parkinson's Laws in
action (the amount on debate on an
issue is inversely proportional to the
issue's actual importance).
Edit (by AM 6/19/2010 11:45): this is also known as "painting the
bikeshed" (or, for short,
"bikeshedding") -- the reference is,
again, to Northcote Parkinson, who
gave "debates on what color to paint
the bikeshed" as a typical example of
"hot debates on trivial topics".
(end-of-Edit).
One syntax difference that I do find
important, and in Python's favor --
but other people will no doubt think
just the reverse -- is "how do you
call a function which takes no
parameters". In Python (like in C),
to call a function you always apply
the "call operator" -- trailing
parentheses just after the object
you're calling (inside those trailing
parentheses go the args you're passing
in the call -- if you're passing no
args, then the parentheses are empty).
This leaves the mere mention of
any object, with no operator involved, as meaning just a reference
to the object -- in any context,
without special cases, exceptions,
ad-hoc rules, and the like. In Ruby
(like in Pascal), to call a function
WITH arguments you pass the args
(normally in parentheses, though that
is not invariably the case) -- BUT if
the function takes no args then simply
mentioning the function implicitly
calls it. This may meet the
expectations of many people (at least,
no doubt, those whose only previous
experience of programming was with
Pascal, or other languages with
similar "implicit calling", such as
Visual Basic) -- but to me, it means
the mere mention of an object may
EITHER mean a reference to the object,
OR a call to the object, depending on
the object's type -- and in those
cases where I can't get a reference to
the object by merely mentioning it I
will need to use explicit "give me a
reference to this, DON'T call it!"
operators that aren't needed
otherwise. I feel this impacts the
"first-classness" of functions (or
methods, or other callable objects)
and the possibility of interchanging
objects smoothly. Therefore, to me,
this specific syntax difference is a
serious black mark against Ruby -- but
I do understand why others would thing
otherwise, even though I could hardly
disagree more vehemently with them:-).
Below the syntax, we get into some
important differences in elementary
semantics -- for example, strings in
Ruby are mutable objects (like in
C++), while in Python they are not
mutable (like in Java, or I believe
C#). Again, people who judge
primarily by what they're already
familiar with may think this is a plus
for Ruby (unless they're familiar with
Java or C#, of course:-). Me, I think
immutable strings are an excellent
idea (and I'm not surprised that Java,
independently I think, reinvented that
idea which was already in Python),
though I wouldn't mind having a
"mutable string buffer" type as well
(and ideally one with better
ease-of-use than Java's own "string
buffers"); and I don't give this
judgment because of familiarity --
before studying Java, apart from
functional programming languages where
all data are immutable, all the languages I knew had mutable strings
-- yet when I first saw the immutable-string idea in Java (which I
learned well before I learned Python),
it immediately struck me as excellent,
a very good fit for the
reference-semantics of a higher level
programming language (as opposed to
the value-semantics that fit best with
languages closer to the machine and
farther from applications, such as C)
with strings as a first-class,
built-in (and pretty crucial) data
type.
Ruby does have some advantages in
elementary semantics -- for example,
the removal of Python's "lists vs
tuples" exceedingly subtle
distinction. But mostly the score (as
I keep it, with simplicity a big plus
and subtle, clever distinctions a
notable minus) is against Ruby (e.g.,
having both closed and half-open
intervals, with the notations a..b and
a...b [anybody wants to claim that
it's obvious which is which?-)], is
silly -- IMHO, of course!). Again,
people who consider having a lot of
similar but subtly different things at
the core of a language a PLUS, rather
than a MINUS, will of course count
these "the other way around" from how
I count them:-).
Don't be misled by these comparisons
into thinking the two languages are
very different, mind you. They aren't. But if I'm asked to compare
"capelli d'angelo" to "spaghettini",
after pointing out that these two
kinds of pasta are just about
undistinguishable to anybody and
interchangeable in any dish you might
want to prepare, I would then
inevitably have to move into
microscopic examination of how the
lengths and diameters imperceptibly
differ, how the ends of the strands
are tapered in one case and not in the
other, and so on -- to try and explain
why I, personally, would rather have
capelli d'angelo as the pasta in any
kind of broth, but would prefer
spaghettini as the pastasciutta to go
with suitable sauces for such long
thin pasta forms (olive oil, minced
garlic, minced red peppers, and finely
ground anchovies, for example - but if
you sliced the garlic and peppers
instead of mincing them, then you
should choose the sounder body of
spaghetti rather than the thinner
evanescence of spaghettini, and would
be well advised to forego the achovies
and add instead some fresh spring
basil [or even -- I'm a heretic...! --
light mint...] leaves -- at the very
last moment before serving the dish).
Ooops, sorry, it shows that I'm
traveling abroad and haven't had pasta
for a while, I guess. But the analogy
is still pretty good!-)
So, back to Python and Ruby, we come
to the two biggies (in terms of
language proper -- leaving the
libraries, and other important
ancillaries such as tools and
environments, how to embed/extend each
language, etc, etc, out of it for now
-- they wouldn't apply to all IMPLEMENTATIONS of each language
anyway, e.g., Jython vs Classic Python
being two implementations of the
Python language!):
Ruby's iterators and codeblocks vs Python's iterators and generators;
Ruby's TOTAL, unbridled "dynamicity", including the ability
to "reopen" any existing class,
including all built-in ones, and
change its behavior at run-time -- vs
Python's vast but bounded
dynamicity, which never changes the
behavior of existing built-in
classes and their instances.
Personally, I consider 1 a wash (the
differences are so deep that I could
easily see people hating either
approach and revering the other, but
on MY personal scales the pluses and
minuses just about even up); and 2 a
crucial issue -- one that makes Ruby
much more suitable for "tinkering",
BUT Python equally more suitable for
use in large production applications.
It's funny, in a way, because both
languages are so MUCH more dynamic
than most others, that in the end the
key difference between them from my
POV should hinge on that -- that Ruby
"goes to eleven" in this regard (the
reference here is to "Spinal Tap", of
course). In Ruby, there are no limits
to my creativity -- if I decide that
all string comparisons must become
case-insensitive, I CAN DO THAT!
I.e., I can dynamically alter the
built-in string class so that
a = "Hello World"
b = "hello world"
if a == b
print "equal!\n"
else
print "different!\n"
end WILL print "equal". In python, there is NO way I can do that.
For the purposes of metaprogramming,
implementing experimental frameworks,
and the like, this amazing dynamic
ability of Ruby is extremely
appealing. BUT -- if we're talking
about large applications, developed by
many people and maintained by even
more, including all kinds of libraries
from diverse sources, and needing to
go into production in client sites...
well, I don't WANT a language that is
QUITE so dynamic, thank you very much.
I loathe the very idea of some library
unwittingly breaking other unrelated
ones that rely on those strings being
different -- that's the kind of deep
and deeply hidden "channel", between
pieces of code that LOOK separate and
SHOULD BE separate, that spells
d-e-a-t-h in large-scale programming.
By letting any module affect the
behavior of any other "covertly", the
ability to mutate the semantics of
built-in types is just a BAD idea for
production application programming,
just as it's cool for tinkering.
If I had to use Ruby for such a large
application, I would try to rely on
coding-style restrictions, lots of
tests (to be rerun whenever ANYTHING
changes -- even what should be totally
unrelated...), and the like, to
prohibit use of this language feature.
But NOT having the feature in the
first place is even better, in my
opinion -- just as Python itself would
be an even better language for
application programming if a certain
number of built-ins could be "nailed
down", so I KNEW that, e.g.,
len("ciao") is 4 (rather than having
to worry subliminally about whether
somebody's changed the binding of name
'len' in the builtins module...).
I do hope that eventually Python does
"nail down" its built-ins.
But the problem's minor, since
rebinding built-ins is quite a
deprecated as well as a rare practice
in Python. In Ruby, it strikes me as
major -- just like the too powerful
macro facilities of other languages
(such as, say, Dylan) present similar
risks in my own opinion (I do hope
that Python never gets such a powerful
macro system, no matter the allure of
"letting people define their own
domain-specific little languages
embedded in the language itself" -- it
would, IMHO, impair Python's wonderful
usefulness for application
programming, by presenting an
"attractive nuisance" to the would-be
tinkerer who lurks in every
programmer's heart...).
Alex

Some others from:
http://www.ruby-lang.org/en/documentation/ruby-from-other-languages/to-ruby-from-python/
(If I have misintrepreted anything or any of these have changed on the Ruby side since that page was updated, someone feel free to edit...)
Strings are mutable in Ruby, not in Python (where new strings are created by "changes").
Ruby has some enforced case conventions, Python does not.
Python has both lists and tuples (immutable lists). Ruby has arrays corresponding to Python lists, but no immutable variant of them.
In Python, you can directly access object attributes. In Ruby, it's always via methods.
In Ruby, parentheses for method calls are usually optional, but not in Python.
Ruby has public, private, and protected to enforce access, instead of Python’s convention of using underscores and name mangling.
Python has multiple inheritance. Ruby has "mixins."
And another very relevant link:
http://c2.com/cgi/wiki?PythonVsRuby
Which, in particular, links to another good one by Alex Martelli, who's been also posting a lot of great stuff here on SO:
http://groups.google.com/group/comp.lang.python/msg/028422d707512283

I'm unsure of this, so I add it as an answer first.
Python treats unbound methods as functions
That means you can call a method either like theobject.themethod() or by TheClass.themethod(anobject).
Edit: Although the difference between methods and functions is small in Python, and non-existant in Python 3, it also doesn't exist in Ruby, simply because Ruby doesn't have functions. When you define functions, you are actually defining methods on Object.
But you still can't take the method of one class and call it as a function, you would have to rebind it to the object you want to call on, which is much more obstuse.

I would like to mention Python descriptor API that allows one customize object-to-attribute "communication". It is also noteworthy that, in Python, one is free to implement an alternative protocol via overriding the default given through the default implementation of the __getattribute__ method.
Let me give more details about the aforementioned.
Descriptors are regular classes with __get__, __set__ and/or __delete__ methods.
When interpreter encounters something like anObj.anAttr, the following is performed:
__getattribute__ method of anObj is invoked
__getattribute__ retrieves anAttr object from the class dict
it checks whether abAttr object has __get__, __set__ or __delete__ callable objects
the context (i.e., caller object or class, and value, instead of the latter, if we have setter) is passed to the callable object
the result is returned.
As was mentioned, this is the default behavior. One is free to change the protocol by re-implementing __getattribute__.
This technique is lot more powerful than decorators.

Ruby has builtin continuation support using callcc.
Hence you can implement cool things like the amb-operator

At this stage, Python still has better unicode support

Python has docstrings and ruby doesn't... Or if it doesn't, they are not accessible as easily as in python.
Ps. If im wrong, pretty please, leave an example? I have a workaround that i could monkeypatch into classes quite easily but i'd like to have docstring kinda of a feature in "native way".

Ruby has a line by line loop over input files (the '-n' flag) from the commandline so it can be used like AWK. This Ruby one-liner:
ruby -ne 'END {puts $.}'
will count lines like the AWK one-liner:
awk 'END{print NR}'
Ruby gets feature this through Perl, which took it from AWK as a way of getting sysadmins on board with Perl without having to change the way they do things.

Ruby has sigils and twigils, Python doesn't.
Edit: And one very important thing that I forgot (after all, the previous was just to flame a little bit :-p):
Python has a JIT compiler (Psyco), a sightly lower level language for writing faster code (Pyrex) and the ability to add inline C++ code (Weave).

My python's rusty, so some of these may be in python and i just don't remember/never learned in the first place, but here are the first few that I thought of:
Whitespace
Ruby handles whitespace completely different. For starters, you don't need to indent anything (which means it doesn't matter if you use 4 spaces or 1 tab). It also does smart line continuation, so the following is valid:
def foo(bar,
cow)
Basically, if you end with an operator, it figures out what is going on.
Mixins
Ruby has mixins which can extend instances instead of full classes:
module Humor
def tickle
"hee, hee!"
end
end
a = "Grouchy"
a.extend Humor
a.tickle » "hee, hee!"
Enums
I'm not sure if this is the same as generators, but as of Ruby 1.9 ruby as enums, so
>> enum = (1..4).to_enum
=> #<Enumerator:0x1344a8>
Reference: http://blog.nuclearsquid.com/writings/ruby-1-9-what-s-new-what-s-changed
"Keyword Arguments"
Both of the items listed there are supported in Ruby, although you can't skip default values like that.
You can either go in order
def foo(a, b=2, c=3)
puts "#{a}, #{b}, #{c}"
end
foo(1,3) >> 1, 3, 3
foo(1,c=5) >> 1, 5, 3
c >> 5
Note that c=5 actually assigns the variable c in the calling scope the value 5, and sets the parameter b the value 5.
or you can do it with hashes, which address the second issue
def foo(a, others)
others[:b] = 2 unless others.include?(:b)
others[:c] = 3 unless others.include?(:c)
puts "#{a}, #{others[:b]}, #{others[:c]}"
end
foo(1,:b=>3) >> 1, 3, 3
foo(1,:c=>5) >> 1, 2, 5
Reference: The Pragmatic Progammer's Guide to Ruby

You can have code in the class definition in both Ruby and Python. However, in Ruby you have a reference to the class (self). In Python you don't have a reference to the class, as the class isn't defined yet.
An example:
class Kaka
puts self
end
self in this case is the class, and this code would print out "Kaka". There is no way to print out the class name or in other ways access the class from the class definition body in Python.

Syntax is not a minor thing, it has a direct impact on how we think. It also has a direct effect on the rules we create for the systems we use. As an example we have the order of operations because of the way we write mathematical equations or sentences. The standard notation for mathematics allows people to read it more than one way and arrive at different answers given the same equation. If we had used prefix or postfix notation we would have created rules to distinguish what the numbers to be manipulated were rather than only having rules for the order in which to compute values.
The standard notation makes it plain what numbers we are talking about while making the order in which to compute them ambiguous. Prefix and postfix notation make the order in which to compute plain while making the numbers ambiguous. Python would already have multiline lambdas if it were not for the difficulties caused by the syntactic whitespace. (Proposals do exist for pulling this kind of thing off without necessarily adding explicit block delimiters.)
I find it easier to write conditions where I want something to occur if a condition is false much easier to write with the unless statement in Ruby than the semantically equivalent "if-not" construction in Ruby or other languages for example. If most of the languages that people are using today are equal in power, how can the syntax of each language be considered a trivial thing? After specific features like blocks and inheritance mechanisms etc. syntax is the most important part of a language,hardly a superficial thing.
What is superficial are the aesthetic qualities of beauty that we ascribe to syntax. Aesthetics have nothing to do with how our cognition works, syntax does.

Surprised to see nothing mentioned of ruby's "method missing" mechanism. I'd give examples of the find_by_... methods in Rails, as an example of the power of that language feature. My guess is that something similar could be implemented in Python, but to my knowledge it isn't there natively.

Another difference in lambdas between Python and Ruby is demonstrated by Paul Graham's Accumulator Generator problem. Reprinted here:
Write a function foo that takes a number n and returns a function that takes a number i, and returns n incremented by i.
Note: (a) that's number, not integer, (b) that's incremented by, not plus.
In Ruby, you can do this:
def foo(n)
lambda {|i| n += i }
end
In Python, you'd create an object to hold the state of n:
class foo(object):
def __init__(self, n):
self.n = n
def __call__(self, i):
self.n += i
return self.n
Some folks might prefer the explicit Python approach as being clearer conceptually, even if it's a bit more verbose. You store state like you do for anything else. You just need to wrap your head around the idea of callable objects. But regardless of which approach one prefers aesthetically, it does show one respect in which Ruby lambdas are more powerful constructs than Python's.

python has named optional arguments
def func(a, b=2, c=3):
print a, b, c
>>> func(1)
1 2 3
>>> func(1, c=4)
1 2 4
AFAIK Ruby has only positioned arguments because b=2 in the function declaration is an affectation that always append.

Ruby has embedded documentation:
=begin
You could use rdoc to generate man pages from this documentation
=end

http://c2.com/cgi/wiki?PythonVsRuby
http://c2.com/cgi/wiki?SwitchedFromPythonToRuby
http://c2.com/cgi/wiki?SwitchedFromRubyToPython
http://c2.com/cgi/wiki?UsingPythonDontNeedRuby
http://c2.com/cgi/wiki?UsingRubyDontNeedPython

In Ruby, when you import a file with
require, all the things defined in
that file will end up in your global
namespace.
With Cargo you can "require libraries without cluttering your namespace".
# foo-1.0.0.rb
class Foo
VERSION = "1.0.0"
end
# foo-2.0.0.rb
class Foo
VERSION = "2.0.0"
end
>> Foo1 = import("foo-1.0.0")
>> Foo2 = import("foo-2.0.0")
>> Foo1::VERSION
=> "1.0.0"
>> Foo2::VERSION
=> "2.0.0"

What do you wish you'd known about when you started learning Python? [closed]

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Closed 10 years ago.
I've decided to learn Python 3. For those that have gone before, what did you find most useful along the way and wish you'd known about sooner?

I learned Python back before the 1.5.2 release, so the things that were key for me back then may not be the key things today.
That being said, a crucial thing that took me a little bit to realize, but I now consider crucial: much functionality that other languages would make intrinsic is actually made available by the standard library and the built-ins.
The language itself is small and simple, but until you're familiar with the built-ins and the "core parts" of the standard library (e.g., nowadays, sys, itertools, collections, copy, ...), you'll be reinventing the wheel over and over. So, the more time you invest in getting familiar with those parts, the smoother your progress will be. Every time you have a task you want to do, that doesn't seem to be directly supported by the language, first ask yourself: what built-ins or modules in the standard library will make the task much simpler, or even do it all for me? Sometimes there won't be any, but more often than not you'll find excellent solutions by proceeding with this mindset.

I wished I didn't know Java.
More functional programming. (see itertools module, list comprehension, map(), reduce() or filter())

List comprehension (makes a list cleanly):
[x for x in y if x > z]
Generator expansion (same as list comprehension but doesn't evaluate until it is used):
(x for x in y if x > z)

Two brain-cramping things. One of which doesn't apply to Python 3.
a = 095
Doesn't work. Why? The leading zero is an octal literal. The 9 is not valid in an octal literal.
def foo( bar=[] ):
bar.append( 1 )
return bar
Doesn't work. Why? The mutable default object gets reused.

What enumerate is for.
That seq = seq.append(item) and seq = seq.sort() both set seq to None.
Using set to remove duplicates.
Pretty much everything in the itertools and collections modules.
How the * and ** prefixes for function arguments work.
How default arguments to functions work internally (i.e. what f.func_defaults is).
How (why, really) to design functions so that they are useful in conjunction with map and zip.
The role of __dict__ in classes.
What import actually does.

Learn how to use iPython
It's got Tab completion.
View all the elements in your namespace with 'whos'.
After you import a module, it's easy to view the code:
>>> import os
>>> os?? # this display the actual source of the method
>>> help() # Python's interactive help. Fantastic!
Most Python modules are well documented; in theory, you could learn iPython and the rest of what you'd need to know could be learned through the same tool.
iPython also has a debug mode, pdb().
Finally, you can even use iPython as a python enabled command line. The basic UNIX commands work as %magic methods. Any commands that aren't magic command can be executed:
>>> os.system('cp file1 file2')

Don't have variable names that are types. For example, don't name a variable "file" or "dict"

Decorators. Writing your own is not something you might want to do right away, but knowing that #staticmethod and #classmethod are available from the beginning (and the difference between what they do) is a real plus.

using help() in the shell on any object, class or path
you can run import code;
code.interact(local=locals()) anywhere in your code and it will start a python shell at that exact point
you can run python -i yourscript.py to start a shell at the end of yourscript.py

Most helpful: Dive Into Python. As a commenter points out, if you're learning Python 3, Dive Into Python 3 is more applicable.
Known about sooner: virtualenv.

That a tuple of a single item must end with a comma, or it won't be interpreted as a tuple.
pprint() is very handy (yes, 2 p's)
reload() is useful when you're re-testing a module while making lots of rapid changes to a dependent module.
And learn as many common "idioms" as you can, otherwise you'll bang your head looking for a better way to do something, when the idiom really is regarded as the best way (e.g. ugly expressions like ' '.join(), or the answer to why there is no isInt(string) function.... the answer is you can just wrap the usage of a "possible" integer with a try: and then catch the exception if it's not a valid int. The solution works well, but it sounds like a terrible answer when you first encounter it, so you can waste a lot of time convincing yourself it really is a good approach.
Those are some things that wasted several hours of my time to determine that my first draft of some code which felt wrong, really was acceptable.
Readings from python.org:
http://wiki.python.org/moin/BeginnerErrorsWithPythonProgramming
http://wiki.python.org/moin/PythonWarts

List comprehensions, if you're coming to Python fresh (not from an earlier version).

Closures. Clean and concise, without having to resort to using a Strategy Pattern unlike languages such as Java

If you learn from a good book, it will not only teach you the language, it will teach you the common idioms. The idioms are valuable.
For example, here is the standard idiom for initializing a class instance with a list:
class Foo(object):
def __init__(self, lst=None):
if lst is None:
self.lst = []
else:
self.lst = lst
If you learn this as an idiom from a book, you don't have to learn the hard way why this is the standard idiom. #S.Lott already explained this one: if you try to make the default initializer be an empty list, the empty list gets evaluated just once (at compile time) and every default-initialized instance of your class gets the same list instance, which was not what was intended here.
Some idioms protect you from non-intended effects; some help you get best performance out of the language; and some are just small points of style, which help other Python fans understand your code better.
I learned out of the book Learning Python and it introduced me to some of the idioms.
Here's a web page devoted to idioms: http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html
P.S. Python code that follows the best-practice Python idioms often is called "Pythonic" code.

I implemented plenty of recursive directory walks by hand before I learned about os.walk()

Lambda functions
http://www.diveintopython.org/power_of_introspection/lambda_functions.html

One of the coolest things I learned about recently was the commands module:
>>> import commands
>>> commands.getoutput('uptime')
'18:24 up 10:22, 7 users, load averages: 0.37 0.45 0.41'
It's like os.popen or os.system but without all of the DeprecationWarnings.
And let's not forget PDB (Python Debugger):
% python -m pdb poop.py

Dropping into interactive mode in IPython
from IPython.Shell import IPShellEmbed
ipshell = IPShellEmbed()
ipshell()

When I started with python, started out with main methods from the examples. This was because I didn't know better, after that I found this on how to create a better main method.

Sequential imports overwrite:
If you import two files like this:
from foo import *
from bar import *
If both foo.py and bar.py have a function named fubar(), having imported the files this way, when you call fubar, fubar as defined in bar.py will be executed. The best way to avoid this is to do this:
import foo
import bar
and then call foo.fubar or bar.fubar. This way, you ALWAYS know which file's definition of fubar will be executed.

Maybe a touch more advanced, but I wish I'd known that you don't use threads to take advantage of multiple cores in (C)python. You use the multiprocessing library.

Tab completion and general readline support, including histories, even in the regular python shell.
$ cat ~/.pythonrc.py
#!/usr/bin/env python
try:
import readline
except ImportError:
print("Module readline not available.")
else:
import rlcompleter
readline.parse_and_bind("tab: complete")
import os
histfile = os.path.join(os.environ["HOME"], ".pyhist")
try:
readline.read_history_file(histfile)
except IOError:
pass
import atexit
atexit.register(readline.write_history_file, histfile)
del os, histfile
and then add a line to your .bashrc
export PYTHONSTARTUP=~/.pythonrc.py
These two things lead to an exploratory programming style of "it looks like this library might do what I want", so then I fire up the python shell and then poke around using tab-completion and the help() command until I find what I need.
Generators and list comprehensions are more useful than you might think. Don't just ignore them.

I wish I knew well a functional language. After playing a bit with Clojure, I realized that lots of Python's functional ideas are borrowed from Lisp or other functional langs

I wish I'd known right off the bat how to code idiomatically in Python. You can pick up any language you like and start coding in it like it's C, Java, etc. but ideally you'll learn to code in "the spirit" of the language. Python is particularly relevant, as I think it has a definite style of its own.
While I found it a little later in my Python career than I would have liked, this excellent article wraps up many Python idioms and the little tricks that make it special. Several of the things people have mentioned in their answers so far are contained within:
Code Like a Pythonista: Idiomatic Python.
Enjoy!

Pretty printing:
>>> print "%s world" %('hello')
hello world
%s for string
%d for integer
%f for float
%.xf for exactly x many decimal places of a float. If the float has lesser decimals that indicated, then 0s are added

I really like list comprehension and all other semifunctional constructs. I wish I had known those when I was in my first Python project.

What I really liked: List comprehensions, closures (and high-order functions), tuples, lambda functions, painless bignumbers.
What I wish I had known about sooner: The fact that using Python idioms in code (e.g. list comprehensions instead of loops over lists) was faster.

That multi-core was the future. Still love Python. It's writes a fair bit of my code for me.

Functional programming tools, like all and any