[Python-ideas] issubclass(collections.OrderedDict, collections.Sequence)

[Ram Rachum]

On Mon, Oct 6, 2014 at 8:35 PM, Guido van Rossum <guido at python.org> wrote:

> On Mon, Oct 6, 2014 at 1:10 AM, Ram Rachum <ram at rachum.com> wrote:
>
>> Here are a couple:
>>
>>  - I'm making a combinatorics package, and a combination space needs to
>> have a __contains__ method that takes a combination and returns whether
>> it's part of a set. Since a combination, unlike a permutation, has no
>> order, I like to have my combinations be canonicalized in a sorted order.
>> For example, in a combination space of 3 items on range(4), (0, 1, 3) would
>> be a combination in that space, but (3, 1, 0) would not because it's not
>> sorted. (It's a k-permutation but not a combination.) However, if the user
>> does `{0, 1, 3} in comb_space` I still want to return True, regardless of
>> whether the set iterator happens to give these items in order or not.
>>
>
> So how are you writing this code today? In the following case, what's in
> the then or else branch?
>
> if not isinstance(x, collections.Ordered):
>     <what???>
> else:
>     <what???>
>
> Even if you could write this, how would you know that an ordered argument
> is in the *canonical* order?
>

That part isn't written yet (the package is still work-in-progress), but
I'm not sure what the problem is. I'll have the code look at `x`. If it's
marked as ordered, then I'd iterate on it. If it has the correct items
(meaning the items of the sequence that this is a combination space of) and
the items in x are in the same order as they are in the sequence, and it
has the correct number of items, then we have a match. If we have `not
isinstance(x, collections.Ordered)` then I do the same thing except I
ignore the order of the items. What's the problem?


>
>  - For the same package, I'm defining `Tally` and `OrderedTally` classes.
>> (Simliar to `Counter` except without having an identity crisis between
>> being a dict subclass and counter; mine are strictly counters.) In the
>> tests I want to easily see whether the class I'm testing is the ordered one
>> or not, so I'll know to run appropriate tests. (There are also
>> `FrozenTally` and `FrozenOrderedTally` so it's not just one class.) I could
>> set `is_ordered = True` on them or give them some base class, but I think a
>> general `collections.Ordered` abstract base class would be the best
>> solution.
>>
>
> Same question.
>
> --
> --Guido van Rossum (python.org/~guido)
>
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