list.extend([]) Question
Dan Brown
seems very counterintuitive to me, at least --- I expected ['a'].extend
([]) to result in ['a'], not None.
Alf P. Steinbach
> Why does extending a list with the empty list result in None? It
> seems very counterintuitive to me, at least --- I expected ['a'].extend
> ([]) to result in ['a'], not None.
It does.
'extend' is an operation that /modifies/ the array.
It just returns None as its expression result, in the same way as e.g. the
Python 3.x 'print' (another pure "doer" operation).
>>> L = ['a']
>>> L
['a']
>>> L2 = L.extend( [] )
>>> L2
>>> L2 is None
True
>>> L
['a']
>>> _
Cheers & hth.,
- Alf
Andre Engels
Extend is a method of the list. The list itself is changed, it does
not return itself:
>>> A = [1,2]
>>> B = [3,4]
>>> C = A.extend(B)
>>> C
>>> C is None
True
>>> A
[1, 2, 3, 4]
Thus, nothing special about extend([]), this is the general behaviour of extend
--
André Engels, andre...@gmail.com
Dan Brown
> It does.
>
> 'extend' is an operation that /modifies/ the array.
>
> It just returns None as its expression result, in the same way as e.g. the
> Python 3.x 'print' (another pure "doer" operation).
>
> >>> L = ['a']
> >>> L
> ['a']
> >>> L2 = L.extend( [] )
> >>> L2
> >>> L2 is None
> True
> >>> L
> ['a']
> >>> _
>
> Cheers & hth.,
>
> - Alf
Aha. Well, I feel a bit silly for not thinking to try it that way.
Thanks!
Steve Holden
How very inconvenient of Python! What it actually does is create an
anonymous list containing only the element 'a', and leave it unchanged
by extending it with an empty list. Since there is no longer any
reference to the list it has become garbage.
Contrast that with:
>>> lst = ['a']
>>> lst.extend([])
>>> lst
['a']
>>> lst.append([])
>>> lst
['a', []]
>>> lst.extend(['1'])
>>> lst
['a', [], '1']
>>>
As you can see by the absence of output, both the .extend() and
.append() list methods return None. They mutate the list instance upon
which they are called.
In your example you were expecting the methods to return the mutated
list. They don't.
regards
Steve
--
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Aahz
http://www.python.org/doc/faq/general/#why-doesn-t-list-sort-return-the-sorted-list
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import antigravity
Gerald Britton
result is whatever the method "extend" returns. But "extend" has no
return value, hence you will see None if you do this interactively.
> --
> http://mail.python.org/mailman/listinfo/python-list
>
--
Gerald Britton
Bruno Desthuilliers
> I think it's because when you do ['a'].extend([]) or whatever, the
> result is whatever the method "extend" returns. But "extend" has no
> return value
It does : it returns None.
Jack Diederich
<gerald....@gmail.com> wrote:
> I think it's because when you do ['a'].extend([]) or whatever, the
> result is whatever the method "extend" returns. But "extend" has no
>
That sums it up. In Python the convention is to raise an exception on
error, return a new value in the case where a new value is created,
and - as in this case - to return None for modification of an existing
value. Returning "None" is vexing if you are used to another language
that has a different convention but is expected for well behaved
python libraries.
So yeah, Python does it that way because the intent is to loudly and
regularly announce that something was modified or to loudly and
regularly announce that something new was /not/ created. It's a
boring story with no whiz-bang feature behind it, but I like that the
language behaves that way for exactly that reason.
-Jack