Nested conditions in python

Question

I am trying to figure out the shortest, most pythonic way to implement a similar to the following syntax:

if A and (B if C):
 print(A)

in a way that:

• if C is False, then B is omitted (therefore (B if C) is True).
• if C is True, then B is evaluated, effectively making the syntax if A and B:

This can be made through various separate if statements, but my ultimate purpose with this was to make it into a list comprehension for a value assignment.

Edit:

The list comprehension I wanted to make was this:

methods = [name for (name, func) in locals().items() \
 if callable(func) and (not __name__ == '__main__' or \
 func.__module__ == __name__)]

So that it returns the function names I have defined in that module as well as if methods is imported from the outside.

• cs95: @chepner Right, thanks, it's not as simple.
• Paritosh Singh: if A and (B if C else True): or (A and B) if C else A:
• mariogarcc: If C is False, then the statement has to be true if A is true. But I admit I hadn't thought of that. @coldpseed Sorry for the late response, I see it was already responded...
• Xion: A, B = True and C = False -> True and A,B,C = True -> True right? If so then it would be if A and not C or B:
• mariogarcc: @RonKalian Ah, I get what you're trying to say, but I meant that I still needed the "print" that I showed in my pseudocode, it's not just about evaluating the expressions.

Answer 1

This should be equivalent, if my old statement logic doesn't fail me =)

if A and (not C or B):
 print(A)

Explanation: "B if C" <=> C -> B <=> not C or B

Expression B is only evaluated if C holds.

Answer 2

your hypothesis:

• if C is False, then B is omitted
• if C is True, then B is evaluated, effectively making the syntax if A and B:

wouldn't that be:

if A and (not C or B):
 print(A)

• if C is false then not C is True and we don't evaluate B
• if C is true, then not C is False, and we have to evaluate B

Answer 3

Your if pseudo-operator is just logical implication, where

C -> B = not C or B

This means you just want

if A and (not C or B):

When C is False, A and (not C or B) == A and (True or B) == A and True == A.

When C is True, A and (not C or B) == A and (False or B) == A and B.

• chepner: Fun, irrelevant fact: some languages actually have logical implication. In Nix, for example, you could actually write A && (C -> B) as a Boolean expression instead of A && (!C || B). Whether that is more readable depends on how grounded you are in logic :)

Answer 4

This:

if A and (B if C else True):
 pass

is closest to your "pseudo code", using the conditional expression x if cond else y in Python. Assuming B=True in case C is False effectively make the if statement consider only the boolean value of A

• chepner: I think this would actually be my preferred way to write it in Python.

Answer 5

I'd probably write it like this:

condition = (A and B) if C else A
if condition:
 print(A)

I've only broken the condition into a separate variable because I think mixing an if with a Python conditional expression looks a little confusing. You'll have to make a call whether it looks confusing when used inside a list comprehension or not.