逻辑AND的评估顺序（python）

Question

In python, which is more efficient:

if a:
    if b:
        # do something

or

if a and b:
    # do something

The latter would be more efficient if b isn't calculated when a is false. But I can't seem to pinpoint whether that's the case in Python docs. Maybe someone can point me to it?

Answer 1

Short-Circuiting

Python doesn't run Y in X and Y if X is false. You can try it out yourself:

if True and print("Hello1"):
    pass

if False and print("Hello2"):
    pass # "Hello2" won't be printed

The reason Python does this is it has something called short-circuiting which optimising logic expressions like this one. Python realises that if if X is false then there is no point in checking Y because the whole expression will be false anyway.

How to Avoid Short-Circuiting

You can bypass this in Python by using the bitwise versions of the logic operators:

and -> & 
or -> |

For example:

if True & bool(print("Hello1")):
    pass

if False & bool(print("Hello2")):
    pass # "Hello2" will be printed this time

Note however that you need to wrap print in bool because bitwise only works on the same data types.

Performance

I would imagine performance wise the one with only one if statement would be faster as other wise python might have to go through 2 if statements if it finds the conditions are true.

Answer 2

This is called short-circuiting, and it is mentioned in the docs .

And the answer is, Python will not evaluate b if a is False.

Answer 3

It evaluates from left to right, simple experiment

def iftrue1():
    print("iftrue1")
    return True

def iftrue2():
    print("iftrue2")
    return True

if iftrue1() and iftrue2():
    pass

This outputs

iftrue1
iftrue2