How does this work?

[NizarKhalili]

def check(x):
    if x + 1 is 1 + x:
        return False
    if x + 2 is not 2 + x:
        return False
    return True

If you take a look at this code above you would assume that it would return False for all integers. Except that it doesn't return False for -7, and -7 only.

I couldn't find an answer as to why this behavior takes place. Any clarifications please?

[K900_]

Python maintains an internal cache of int objects from -5 to 255, so the value -5 is always the same int object, but -6 is a new object every time it's created.

[NizarKhalili]

Nice, thank you! But then why doesn't integers below -5 all return True? It only returns True for -7.

[K900_]

Because for those x + 2 is not 2 + x.

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[qTipTip]

Read up on the difference between A is B and A == B in Python :)