Develop Reference

How do the for / while / print *things* work in python?

What i mean is, how is the syntax defined, i.e. how can i make my own constructs like these?
I realise in a lot of languages, things like this will be built into the compiler / spec, and so it's dealt with by the compiler (at least that how i understand it to work).
But with python, everything i've come across so far has been accessible to the programmer, and so you more or less have the freedom to do whatever you want.
How would i go about writing my own version of for or while? Is it even possible?
I don't have any actual application for this, so the answer to any WHY?! questions is just "because why not?" or "curiosity".

No, you can't, not from within Python. You can't add new syntax to the language. (You'd have to modify the source code of Python itself to make your own custom version of Python.)
Note that the iterator protocol allows you to define objects that can be used with for in a custom way, which covers a lot of the possible use cases of writing your own iteration syntax.

Well, you have a couple of options for creating your own syntax:
Write a higher-order function, like map or reduce.
Modify python at the C level. This is, as you might expect, relatively easy as compared with fiddling with many other languages. See this article for an example: http://eli.thegreenplace.net/2010/06/30/python-internals-adding-a-new-statement-to-python/
Fake it using the debug facilities, or the encodings facility. See this code: http://entrian.com/goto/download.html and http://timhatch.com/projects/pybraces/
Use a preprocessor. Here's one project that tries to make this easy: http://www.fiber-space.de/langscape/doc/index.html
Use of the python facilities built in to achieve a similar effect (decorators, metaclasses, and the like).
Obviously, none of this is quite what you're looking for, but python, unlike smalltalk or lisp, isn't (necessarily) programmed in itself and guarantees to expose its own underlying execution and parsing mechanisms at runtime.

You can't make equivalent constructs. for, while, if etc. are statements, and they are built into the language with their own specific syntax. There are languages that do allow this sort of thing though (to some degree), such as Scala.

while, print, for etc. are keywords. That means they are parsed by the python parser whilst reading the code, stripped any redundant characters and result in tokens. Afterwards a lexer takes those tokens as input and builds a program tree which is then excuted by the interpreter. Said so, those constructs are used only as syntactic sugar for underlying lexical machinery and as such are not visible from inside the code.

Translating code from Python to Smalltalk

Let me say first that I'm NOT searching for automagical solutions here. I want to translate code from Python to Smalltalk because I've noticed some very simple sentences can be automatically translated, examples:
Assigning a variable to a value
Python
i = 1
Smalltalk
i := 1.
Creating a new instance of a class
Python
instance = module.ClassName()
Smalltalk
instance := ClassName new.
A for loop
Python
for a in [0,1,2]:
print (str(a)+str(a))
Smalltalk
#(0 1 2) do: [: a | Transcript show: a + a; cr ]
and so on (while loops, conditionals, etc). My idea is to have a tool which translates all this extremely "simple" cases, and then I may complete or teach a rule system by hand.
Do you know any programming translation tool or library which can help me?
In case you haven't heard of any tool, what technique/pattern you will use to implement such translation? Can you provide a link to an example?
Thanks

You need to parse the Python code, walk the abstract syntax tree that is generated by the parser and output your Smalltalk. There's a nice article about Python ASTs by Eli Bendersky and a slightly older one here. Python makes this relatively straight forward as the Python standard library exposes a lot of the internal tooling of the interpreter and the documentation is reasonably comprehensive.

I am not aware of any such tool, and in general case it might be complicated and/or inefficient. So your route would depend on your more precise need: porting an existing python library, just using it from smalltalk, or making nice clean smalltalk code that does the same thing as python one.
Routes I would consider:
leaving python library as is, and calling it from smalltalk through c interface
implementing python parser in pettit parser an then:
implement smalltalk generator maybe assisted by human through user interface
python interpreter in smalltalk
Note that generator variant might face some difficult issues in general cases, for instance smalltalk has fixed number of instance variables, while in python you can attach then as you go. You could work around that, but resulting smalltalk code might not be pretty.
As for implementing python inside smalltalk take a look at the helvetia presentation from Lukas Renggli, it is on the subject of including other languages inside smalltalk IDE.

Take a look at ply, which is a Lex-Yacc Python implementation. I've used it mostly for translating some other language into Python byte code by building a Python AST with it, but the opposite should be also possible.

Python modding - prevent dangerous scripts to be imported?

I want to allow users to make their own Python "mods" for my game, by placing their scripts in a special folder which the game "scans" for Python modules and imports.
What would be the simplest way to prevent "dangerous" scripts from being imported? I don't want people complaining to me that they used someone's mod and it erased their hard drive.
Things I would like to limit is accessing/modifying/creating any files outside of their folder and connecting to the internet/downloading/sending data. If you can thik of anything else, let me know.
So how can this be done?

Restricted Python seems to able to restrict functionality for code in a clean way and is compatible with python up to 2.7.
http://pypi.python.org/pypi/RestrictedPython/
e.g.
By supplying a different __builtins__ dictionary, we can rule out unsafe operations, such as opening files [...]

The obvious way to do it is to load the module as a string and exec it. This has just as many security risks, but might be easier to block by using custom globals and locals. Have a look at this question - it gives some really good guidance on this. As pointed out in Delnan's comments, this isn't completely secure though.
You could also try this. I haven't used it, but it seems to provide a safe environment for unsafe scripts.

There are some serious shortcomings for sandboxed python execution. aquavitae's answer links to some good discussion on the matter, especially this blog post. Read that first.
There is a kernel of secure execution within cPython. The fundamental idea is to replace the __builtins__ global (Note: not the __builtin__ module), which informs python to turn on some security features; making a handful of attributes on certain objects inaccessible, and removing most of the implementation objects from the interpreter when evaulating that bit of code.
You'll then need to write an actual implementation; in such a way that the protected modules are not the leaked into the sandbox. A fairly tested "file" replacement is provided in the linked blog. Getting a look on that might give you an idea of how involved and complex this problem is.
So now that you have understood that this is a challenge in python; you should take a look at languages with sandbox execution as a core feature, such as Lua, which is very popular in games.

Giving them python execution and trying to limit what they do is asking for trouble. See this SO question for discussion and a pointer to a good article. (You would presumably disable "eval", but it wouldn't make much difference in practice.
My suggestion: Turn the question around. Your goal is to provide them with scripting facilities so they can enhance the game. Find or define an interpreter for a suitable scripting language that has the features you need, and use it to execute their scripts. For example, you could support data persistence in a simple keystore model, without giving them file creation access. Or give them a command to create files but ensure it only accepts a path-less filename. The essential thing is to ensure that there is NO way for them to execute python commands directly.

Why is it not possible to create a practical Perl to Python source code converter?

It would be nice if there existed a program that automatically transforms Perl code to Python code, making the resultant Python program as readable and maintainable as the original one, let alone working the same way.
The most obvious solution would just invoke perl via Python utils:
#!/usr/bin/python
os.exec("tail -n -2 "+__file__+" | perl -")
...the rest of file is the original perl program...
However, the resultant code is hardly a Python code, it's essentially a Perl code. The potential converter should convert Perl constructs and idioms to easy-to-read Python code, it should retain variable and subroutine names (i.e. the result should not look obfuscated) and should not shatter the wrokflow too much.
Such a conversion is obviously very hard. The hardness of the conversion depends on the number of Perl features and syntactical constructs, which do not have easy-to-read, unobfuscated Python equivalents. I believe that the large amount of such features renders such automatic conversion impossible practically (while theoretical possibility exists).
So, could you please name Perl idioms and syntax features that can't be expressed in Python as concise as in the original Perl code?
Edit: some people linked Python-to-Perl conventers and deduced, on this basis, that it should be easy to write Perl-to-Python as well. However, I'm sure that converting to Python is in greater demand; still this converter is not yet written--while the reverse has already been! Which only makes my confidence in impossibility of writing a good converter to Python more solid.

Your best Perl to Python converter is probably 23 years old, just graduated university and is looking for a job.

Why Perl is not Python.
Perl has statements which Python more-or-less totally lacks. While you can probably contrive matching statements, the syntax will be so utterly unlike Perl as to make it difficult to call it a "translation". You'd really have to cook up some fancy Python stuff to make it as terse as the original Perl.
Perl has run-time semantics which are so unlike Python as to make translation very challenging. We'll look at just one example below.
Perl has data structures which are enough different from Python that translation is hard.
Perl threads don't share data by default. Only selected data elements can be shared. Python threads have more common "shared everything" data.
One example of #2 should be enough.
Perl:
do_something || die()
Where do_something is any statement of any kind.
To automagically translate this into Python you'd have to wrap every || die() statement in
try:
python_version_of_do_something
except OrdinaryStatementFailure, e:
die()
sys.exit()
Where the more common formulation
Perl
do_something
Would become this using simple -- unthinking -- translation of the source
try:
python_version_of_do_something
except OrdinaryStatementFailure, e:
pass
And, of course,
Perl
do_this || do_that || die()
Is even more complex to translate into Python.
And
Perl
do_this && do_that || die()
really push the envelope. My Perl is rusty, so I can't recall the precise semantics of this kind of thing. But you have to totally understand the semantics to work out a Pythonic implementation.
The Python examples are not good Python. To write good Python requires "thinking", something an automatic translated can't do.
And every Perl construct would have to be "wrapped" like that in order to get the original Perl semantics into a Pythonic form.
Now, do a similar analysis for every feature of Perl.

Just to expand on some of the other lists here, these are a few Perl constructs that are probably very clumsy in python (if possible).
dynamic scope (via the local keyword)
typeglob manipulation (multiple variables with the same name)
formats (they have a syntax all their own)
closures over mutable variables
pragmas
lvalue subroutines (mysub() = 5; type code)
source filters
context (list vs scalar, and the way that called code can inspect this with wantarray)
type coercion / dynamic typing
any program that uses string eval
The list goes on an on, and someone could try to create a mapping between all of the analogous constructs, but in the end it will be a failure for one simple reason.
Perl can not be statically parsed. The definitions in Perl code (particularly those in BEGIN blocks) change the way the compiler is going to interpret the remaining code. So for non-trivial programs, conversion from Perl => Python suffers from the halting problem.
There is no way to know exactly how all of the program will be compiled until the program has finished running, and it is theoretically possible to create a Perl program that will compile differently every time it is run. Meaning that one Perl program could map to an infinite number of Python programs, the correct of which is only know after running the original program in the perl interpreter.

It is not impossible, it would just take a lot of work.
By the way, there is Perthon, a Python-to-Perl translator. It just seems like nobody is willing to make one that goes the other way.
EDIT: I think I might I've found the reason why a Python to Perl translator is much easier to implement. It's because Python lets you fiddle with a script's AST. See parser module.

Perl can experimentally be built to collect additional information (for instance, comments) during compilation of perl code and even emit the results as XML. There doesn't appear to be any documentation of this outside the source, except for: http://search.cpan.org/perldoc/perl5100delta#MAD
This should be helpful in building a translator. I'd expect you to get 80% of the way there fairly easily, 95% with great difficulty, and never much better than that. There are too many things that don't map well.

Fundamentally, these are two different languages. Converting from one to another and have the result be mostly readable would mean that the software would have to be able to recognize and generate code idioms, and be able to do some static analysis.
The meaning of a program may be exactly defined by the language definition, but the programmer did not necessarily require all the details. A C programmer testing if the value a printf() returned is negative is checking for an error condition, and doesn't typically care about the exact value. if (printf("%s","...") < 0) exit(); can be translated into Perl as print "..." or die();. These statements may not mean exactly the same thing, but they'll typically be what the programmer means, and to create idiomatic C or Perl code from idiomatic Perl or C code the translator must take this into account.
Since different computer languages tend to have different slightly semantics for similar things, it's typically impossible to translate one language into another and come up with the exact same meaning in readable form. To create readable code, the translator needs to understand what the programmer was intending to do, and that's real difficult.
In addition, it would be easier to translate from Python to Perl rather than Perl to Python. Python is intended as a straightforward language with clear standard ways to do things, while Perl is an unduly complex language with the motto "There's More Than One Way To Do It." Translating a Python expression into one of the innumerable corresponding Perl expressions is easier than figuring out what the Perl programmer meant and expressing it in Python.

Python scope and namespace are different from Perl.
In Python, everything is an object. In Perl, everything under the hood seems to be a list/hash/scalar/reference/function. This induces different design approaches and idioms.
Perl has anonymous code blocks and can generate closures on the fly with some branches. I am pretty sure that is not a python feature.
I do think that a very smart chap could statically analyze the bulk of Perl and produce a program that takes small Perl programs and output Python programs that do the same job.
I am much more doubtful about the feasibility of large and/or gnarly Perl translation. Some of us write some really funky code at times.... :)

This is impossible just because you can't even properly parse perl code. See Perl Cannot Be Parsed: A Formal Proof for more details.

The B set of modules by Malcolm Beattie would be the only sane starting point for something like this, though I'm with other answers in that this would be a difficult problem to solve. In general, translating the sense of one high-level language into another high-level language requires a high-level translator, and, for the time being, that can mean only a human.
The difficulty of this problem, for any pair of languages, is due to fundamental differences in the nature of the languages in question, such as runtime semantics and common idioms, not to mention libraries.

The reason it is close to impossible to create a generic translator from one high-level language to another, is that the program only describe HOW and not WHY (this is the reason for comments in the source code).
In order to create a meaningful program in another highlevel language you (or the translator program) needs to know WHY to be able to create the best possible program. If you cannot do that, all you can do is essentially to create a Python interpreter for the compiled version of the Perl program.
In other words, to do this properly you need to go outside the box, and this is very hard for a computer.

NullUserException basically summed it up - it certainly can be done; it would just be an enormous amount of effort to do so. Some language conversion utilities I've seen compile to an intermediate language (such as .NET's CIL) and then decompile that to the desired language. I have not seen any for Perl to Python. You can, however, find a Python to Perl converter here, though that's likely of little use to you unless you're trying to create your own, in which case it may provide some helpful reference.
Edit: if you just need the exact functionality in a Python script, PyPerl may be of some use to you.

Try my version of the Pythonizer: http://github.com/snoopyjc/pythonizer - it does a decent job

Making your own statements

Is there a way to define new statements like def, with, for of my own in Python? Of course, I don't mean to override the existing statements, only create some of my own.
If so, how do I do it? Can you point me to good docs on the subject?

No, you cannot add new syntax within a Python program. The only way to alter the language is to edit and recompile the grammar file and supporting C code, to obtain a new altered interpreter, compiler and runtime.

You can't (re)define language keywords without rewriting a compiler/interpreter/etc. What you could do perhaps is write a something like a DSL (domain-specific language) and something that translates your keyword statements into proper python statements, which might be an easier route.

While you can't modify the syntax of Python itself (without recompiling as Alex has mentioned), you can use metaprogramming techniques. Below is a link to a presentation on creating a DSL in Python.
http://blog.brianbeck.com/post/53538107/python-dsl-i
If you're not married to Python, Ruby is a great language for defining DSL's, as it has broader metaprogramming capabilities.
http://www.themomorohoax.com/2009/02/25/how-to-write-a-clean-ruby-dsl-rails

Ren'Py is an example of an extension for Python that allows custom statements by implementing its own parser and compiler.

There are programming languages that let you do this (Tcl, for example), but Python isn't one of those languages.