Puzzled by list-appending behavior

[MRAB]

On 26/05/2011 06:17, Chris Rebert wrote:
> On Wed, May 25, 2011 at 9:46 PM, Uncle Ben<bgreen at nycap.rr.com>  wrote:
>> In playing with lists of lists, I found the following:
>>
>> (In 3.1, but the same happens also in 2.7)
>>
>> list = [1,2,3]
>> list.append ( [4,5,6] )
>
> Note the lack of output after this line. This indicates that
> list.append([4,5,6]) returned None. Contrast this with, say,
> list.pop().
>
>> x = list
>> x   ->
>>     [1,2,3,[4,5,6]]
>> as expected.
>>
>> But the shortcut fails:
>>
>> list=[1,2,3]
>> x = list.append( [4,5,6] )
>> x   ->
>>    nothing
>>
>> Can someone explain this to me?
>
> The append() method does *not* return the now-appended-to list. It is
> a mutator method that modifies the list object in-place; per
> convention, it therefore returns None to reinforce its side-effecting
> nature to the user (the interactive interpreter by default does not
> display None expression results); analogous methods in other languages
> return void.
>
> list.remove(), list.sort(), and list.extend() similarly return None
> rather than the now-modified list.
>
I'd just like to point out that it's a convention, not a rigid rule.
Sometimes it's not followed, for example, dict.setdefault.