# [Python-ideas] Consider making enumerate a sequence if its argument is a sequence

Brett Cannon brett at python.org
Wed Sep 30 18:43:12 CEST 2015

```On Wed, 30 Sep 2015 at 09:38 Neil Girdhar <mistersheik at gmail.com> wrote:

> In fairness, one is a superset of the other.  You always get an Iterable.
> You sometimes get a Sequence.  It's a bit like multiplication? with
> integers you get integers, with floats, you get floats.
>

No, it's not like multiplication. =) I hate saying this since I think it's
tossed around too much, but int/float substitution doesn't lead to a Liskov
substitution violation like substituting out a sequence for an iterator
(which is what will happen if the type of the argument to `enumerate`
changes). And since you can just call `list` or `tuple` on enumerate and
get exactly what you're after without potential bugs cropping up if you
don't realize from afar you're affecting an assumption someone made, I'm
-1000 on this idea.

-Brett

>
> On Wed, Sep 30, 2015 at 12:35 PM Brett Cannon <brett at python.org> wrote:
>
>> On Wed, 30 Sep 2015 at 08:28 Neil Girdhar <mistersheik at gmail.com> wrote:
>>
>>> What are the pros and cons of making enumerate a sequence if its
>>> argument is a sequence?
>>>
>>> I found myself writing:
>>>
>>>                 for vertex, height in zip(
>>>                         self.cache.height_to_vertex[height_slice],
>>>                         range(height_slice.start, height_slice.stop)):
>>>
>>> I would have preferred:
>>>
>>>                 for height, vertex in enumerate(
>>>                         self.cache.height_to_vertex)[height_slice]:
>>>
>>
>> Because you now suddenly have different types and semantics of what
>> enumerate() returns based on its argument which is easy to mess up if
>> self.cache.height_to_vertex became an iterator object itself instead of a
>> sequence object. It's also not hard to simply do `tuple(enumerate(...))` to
>> get the exact semantics you want: TOOWTDI.
>>
>> IOW all I see are cons. =)
>>
>
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