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List comprehension syntax - how to use ... if... else? [duplicate]

This question already has answers here:
if/else in a list comprehension
(12 answers)
Is it possible to use 'else' in a list comprehension? [duplicate]
(6 answers)
Closed 3 years ago.
I'm just getting started with Python and was reading about list comprehensions.
The following code:
my_list = [x ** x for x in range(1,11) if x % 2 == 0]
print(my_list)
... produces this output:
[4, 256, 46656, 16777216, 10000000000]
I then tried this code:
my_list = [x ** x for x in range(1,11) if x % 2 == 0 else 7]
print(my_list)
... but I got a syntax error starting at the second "e" in else.
Can someone explain why I'm getting a syntax error? I'd like to have the list create a list of even squares based on the value of the base (x), and to have the value "49" if the list value is not an "even number" square.

In this case, you would want
my_list = [x ** x if x % 2 == 0 else 49 for x in range(1,11)]
If you use an if at the end of a list comprehension like that, it is used to select which elements to include. If you want a different result depending on the case, you need to use a ternary if at the beginning instead.
For more info, take a look at the docs here. To quote:
A list comprehension consists of brackets containing an expression followed by a for clause, then zero or more for or if clauses. The result will be a new list resulting from evaluating the expression in the context of the for and if clauses which follow it.
There is no support for else here, presumably because a ternary if (if condition then foo else bar) in the 'body' of the comprehension already accomplishes this and "There should be one-- and preferably only one --obvious way to do it."

Find which variable equals a value [duplicate]

This question already has answers here:
How do I create variable variables?
(17 answers)
Closed 6 years ago.
I have a list of variables in Python:
a = 1
b = 0
c = 0
These come from part of a script that gets user input for certain fields to process, so 1 means "do something to 'a'" and 0 means "do not include." So it would be useful to know which fields need to be used later in the script, i.e. which of them equal 1.
What's the best way to find which of a, b or c is equal to 1?
I tried this method (from here):
test_list = [a,b,c]
next((x for x in test_list if x == 1), None)
Which returns 1, but I'm looking for it to return a. This is precondition, the next step in the script takes the variables that equal 1 and does something with them, by variable name rather than value.
I could do it long-hand like this:
if a == 1:
print "a"
if b == 1:
...
but I suspect there's a nice Pythonic way to find the name of a variable from a list of variables that equals a specific value.

You should consider using a dictionary (a mapping) instead of a list, as names only reference objects but are not the objects themselves:
d = dict(a = 1, b = 2, c = 0)
print(next((k for k, v in d.items() if v == 1), None))
# a

what is difference between [None] and [] in python? [duplicate]

This question already has answers here:
What is a None value?
(9 answers)
Is there a difference between "==" and "is"?
(13 answers)
Closed 6 years ago.
I think [None] is same as [], but in my test , maybe there is something...
>>>print len([])
0
>>>print len([None])
1
when should i use the None ? and []
and another interesting question
>>>c= []
>>>d= []
>>>print c is d
False
>>>a= 1
>>>b=1
print a is b
True
why empty list's id granting that is different?

[] is an empty list
[None] is a list with one element. That one element is None
is checks for reference equality. If both objects refer to the same object by reference then is will return true.
a = []
b = a
a is [] #false
a is b #true

[None] does not mean that there is nothing in the list. None is a keyword in python which has a special meaning. It is like NIL or NULL in other languages.
When you say [None], you are saying "I would like to have a list which contains the special object called None". This is different than saying "I would like a list which contains no elements" (by typing []).

Question 1:
None is an object. It is of type "NoneType".
This can be seen by doing something like this in the terminal:
>>> type(None)
<type 'NoneType'>
So, when you put this object in a list, the list has one element.
Question 2:
The assignment operator in Python, =, is used to attach a name to an object. In the case of immutable objects, like integers, multiple names can be attached to the same object. That is what you are doing with a and b. So, when you test their identity, using the is operator, you see that the two names point to the identical object.
Alternatively, when you attach a name to a newly created list (which you created with the [] operator) it is a different list each time.

None is a valid element, but you can treat it like a stub or placeholder. So it counts as an element inside the list even if there is only a None.
For (equality) comparisons you shouldn't use is. Use ==!
Because is can lead to strange behaviour if you don't know exactly when and how to use it. For example:
>>> 1900 is 1900
True
>>> a = 1900
>>> b = 1900
>>> a is b
False
>>> a, b = 1900, 1900
>>> a is b
True
This rather strange behaviour is explained for example in this question: Why does Python handle '1 is 1**2' differently from '1000 is 10**3'?
This won't happen when you use ==:
>>> a == b
True
>>> 1900 == 1900
True
like one would expect.

You want to use None to imply that there is no valid object. You want to use [] to imply an object that is of type list and has no elements.
[None] is a list with one element which is None
>>>c= [] # This is a new list object
>>>d= [] # This is another new list object
In Python , x is y is used to check whether x and y are the same objects.
Here, c and d point to different list objects.
so,
>>>print c is d
False
is expected.
On the other hand,
>>>c= [] # This is a new list object
>>>d = c # This is the same object as c
>>>print c is d
True
Here, a and b are primitives, not objects
>>>a= 1
>>>b=1
So, this is expected:
print a is b
True

Checking if two lists share at least one element [duplicate]

This question already has answers here:
Test if lists share any items in python
(9 answers)
Closed 4 years ago.
I have two lists, for example:
a = ["mail1", "mail2", "mail3", "mail4"]
b = ["mail2", "mail5"]
and I want to check if any of the elements in list b also appears in list a.
I wanted to know if there is a way (and what is it) to do this without a for loop.
Also I wanted to know how can I create a list of boolean values, where each value will be the result of the comparison of values a[i] and b[i], something like:
[z for i, j in zip(a, b) z = i == j] # (just with the right syntax)
z will be 1 if in some place i == j, so I can check the array for any 'True' values.

You can use any:
any(x in a for x in b)
The nice thing about this generator expression is that any will return True as soon as the generator yields a True, i.e. there won't be redundant x in a lookups.
Edit:
You can improve the time complexity by making a set from a.
a_set = set(a)
any(x in a_set for x in b)
Regarding your new question:
[x == y for x,y in zip(a,b)]

Elegant way is to use sets:
a = ["mail1", "mail2", "mail3", "mail4"]
b = ["mail2", "mail5"]
anb = set(a) & set(b)
print anb
if anb:
print True
>>> set(['mail2'])
>>> True

The any function takes in an iterable (See documentation here), so the answer should be any([x in a for x in b])

Meaning of using commas and underscores with Python assignment operator? [duplicate]

This question already has answers here:
What is the purpose of the single underscore "_" variable in Python?
(5 answers)
Closed 8 months ago.
Reading through Peter Norvig's Solving Every Sudoku Puzzle essay, I've encountered a few Python idioms that I've never seen before.
I'm aware that a function can return a tuple/list of values, in which case you can assign multiple variables to the results, such as
def f():
return 1,2
a, b = f()
But what is the meaning of each of the following?
d2, = values[s] ## values[s] is a string and at this point len(values[s]) is 1
If len(values[s]) == 1, then how is this statement different than d2 = values[s]?
Another question about using an underscore in the assignment here:
_,s = min((len(values[s]), s) for s in squares if len(values[s]) > 1)
Does the underscore have the effect of basically discarding the first value returned in the list?

d2, = values[s] is just like a,b=f(), except for unpacking 1 element tuples.
>>> T=(1,)
>>> a=T
>>> a
(1,)
>>> b,=T
>>> b
1
>>>
a is tuple, b is an integer.

_ is like any other variable name but usually it means "I don't care about this variable".
The second question: it is "value unpacking". When a function returns a tuple, you can unpack its elements.
>>> x=("v1", "v2")
>>> a,b = x
>>> print a,b
v1 v2

The _ in the Python shell also refers to the value of the last operation. Hence
>>> 1
1
>>> _
1
The commas refer to tuple unpacking. What happens is that the return value is a tuple, and so it is unpacked into the variables separated by commas, in the order of the tuple's elements.

You can use the trailing comma in a tuple like this:
>>> (2,)*2
(2, 2)
>>> (2)*2
4