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Date: Wed, 25 May 2011 22:17:45 -0700
Subject: Re: Puzzled by list-appending behavior
From: Chris Rebert <clp2@rebertia.com>
To: Uncle Ben <bgreen@nycap.rr.com>
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On Wed, May 25, 2011 at 9:46 PM, Uncle Ben <bgreen@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 =3D [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 =3D list
> x =C2=A0 ->
> =C2=A0 =C2=A0[1,2,3,[4,5,6]]
> as expected.
>
> But the shortcut fails:
>
> list=3D[1,2,3]
> x =3D list.append( [4,5,6] )
> x =C2=A0 ->
> =C2=A0 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.

Cheers,
Chris
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http://rebertia.com