Question

These work:

>>> print [1,2,3] + [4,5]
[1,2,3,4,5]
>>> 'abc'.upper()
'ABC'

This doesn’t:

>>> print [1,2,3].extend([4,5])
None

Why? You can use string methods on bare strings, so why can’t you use methods on bare sequence types such as lists? Even this doesn’t work:

>>> print list([1,2,3]).extend([4,5])
None

N.B. For a colloquial meaning of ‘work’. Of course there’ll be a good reason why my expected behaviour is incorrect. I’m just curious what it is.

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P.S. I’ve accepted soulcheck’s answer below, and he is right, but on investigating how the addition operator is implemented I just found that the following works:

>>> [1,2,3].__add__([4,5])
[1, 2, 3, 4, 5]

But presumably the add method doesn’t modify the underlying object and creates a new one to return, like string methods.

Answer 1

extend works, just doesn’t return any value as it’s modifying the object it works on. That’s a python convention to not return any value from methods that modify the object they’re called on.

Try:

 a = [1,2,3]
 a.extend([4,5])
 print a

edit:

As for why one reason for this could be that it’s sometimes ambigous what the method should return. Should list.extend return the extended list? A boolean? New list size?

Also it’s an implementation of command-query separation, ie. if it returned something it would be a query and command in one and cqs says it’s evil (although sometimes handy – see list.pop()).

So generally you shouldn’t return anything from mutators, but as with almost everything in python it’s just a guideline.

list.__add__() is the method called when you do + on lists – it returns a new list.