On 26/05/2011 06:17, Chris Rebert wrote:
On Wed, May 25, 2011 at 9:46 PM, Uncle Ben<bgr...@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.
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