Question

The analogue for strings holds true:

string1 = 'abc'
''.join(string1) == string1 # True

So why doesn’t this hold true:

list1 = ['a', 'b', 'c']
[].extend(list1) == list1 # AttributeError: 'NoneType' object has no attribute 'extend'

type([]) returns list. Why would it get perceived as a NoneType instead of a list which would have the extend method?

This is an academic question. I wouldn’t do this is regular code, I just want to understand.

Answer 1

Because list.extend() modifies the list in place and does not return the list itself. What you’d need to do to get what you expect is:

lst = ['a', 'b', 'c']
cplst = []
cplst.extend(lst)
cplst == lst

The functions you reference are not really analogous. join() returns a new string created by concatenating the members of an iterator together with the string being joined on. An analogous list operation would look more like:

def JoiningList(list):

    def join(self, iterable):
        new_list = iterable[0]
        for item in iterable[1:]:
            new_list.extend(self)
            new_list.append(item)
        return new_list