# list.extend([]) Question

Discussion in 'Python' started by Dan Brown, Jan 30, 2010.

1. ### Dan BrownGuest

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.

Dan Brown, Jan 30, 2010

2. ### Alf P. SteinbachGuest

* Dan Brown:
> 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

Alf P. Steinbach, Jan 30, 2010

3. ### Andre EngelsGuest

On Sat, Jan 30, 2010 at 4:32 PM, Dan Brown <> wrote:
> Why does extending a list with the empty list result in None? Â It
> seems very counterintuitive to me, at least --- I expected ['a'].extend
> ([]) to result in ['a'], not None.

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

--

Andre Engels, Jan 30, 2010
4. ### Dan BrownGuest

On Jan 30, 8:38 am, "Alf P. Steinbach" <> wrote:
> 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!

Dan Brown, Jan 30, 2010
5. ### Steve HoldenGuest

Dan Brown wrote:
> 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.

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
--
Steve Holden +1 571 484 6266 +1 800 494 3119
PyCon is coming! Atlanta, Feb 2010 http://us.pycon.org/
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Steve Holden, Jan 30, 2010
6. ### AahzGuest

Aahz, Feb 5, 2010
7. ### Gerald BrittonGuest

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, hence you will see None if you do this interactively.

On Fri, Feb 5, 2010 at 10:55 AM, Aahz <> wrote:
> In article <>,
> Dan Brown  <> wrote:
>>
>>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.

>
> http://www.python.org/doc/faq/general/#why-doesn-t-list-sort-return-the-sorted-list
> --
> Aahz ()           <*>         http://www.pythoncraft.com/
>
> import antigravity
> --
> http://mail.python.org/mailman/listinfo/python-list
>

--
Gerald Britton

Gerald Britton, Feb 5, 2010
8. ### Bruno DesthuilliersGuest

Gerald Britton a écrit :
> 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.

Bruno Desthuilliers, Feb 5, 2010
9. ### Jack DiederichGuest

On Fri, Feb 5, 2010 at 11:31 AM, Gerald Britton
<> 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
> return value, hence you will see None if you do this interactively.
>

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

Jack Diederich, Feb 5, 2010