问题
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]
回答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]+[].
回答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.
来源：https://stackoverflow.com/questions/549741/python-extend-with-an-empty-list-bug