Python extend with an empty list bug? [duplicate]

Question

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• Why does list.append evaluate to false in a boolean context? [duplicate] 7 answers

Why does python 2.5.2 have the following behavior

>>>[2].extend([]) == [2]
False

>>> [2].extend([]) == None
True

$ python --version
Python 2.5.2

I assume I'm not understanding something here, but intuitively I'd think that [2].extend([]) should yield [2]

Answer 1

Extend is a method of list, which modifies it but doesn't return self (returning None instead). If you need the modified value as the expression value, use +, as in [2]+[].

Answer 2

Exactly.

>>> x = [2]
>>> x.extend([]) # Nothing is printed because the return value is None
>>> x == [2]
True
>>> x
[2]

They do this on purpose so that you will remember that the extend function is actually modifying the list in-place. Same with sort(). It always returns None.