[Python-ideas] Adding collections.abc.Ordered

Mon Nov 9 20:03:11 EST 2015

If my thoughts got lost in the mess of text: No, I don't think it is
appropriate.

On Mon, Nov 9, 2015 at 7:47 PM Guido van Rossum <guido at python.org> wrote:

> Hm, this just indicates whether some UI appears by which the user can
> reorder the forms in a list? Is it even appropriate to move this from the
> instance to the class?
>
> On Mon, Nov 9, 2015 at 4:20 PM, Michael Selik <mike at selik.org> wrote:
>
>> I found a use case, out in the wild!
>>
>> I've been searching through GitHub for "ordered" and similar phrases.
>> Much of the usage is OrderedDict and OrderedSet, which really don't
>> benefit from an essentially redundant inheritance from a hypothetical
>> collections.Ordered. There are some tools that enable UI reordering,
>> and a bunch of tree- and graph-traversal ordering functions. Again,
>> not much benefit from a collections.Ordered.
>>
>> However, Django has a class factory that creates an attribute `can_order`:
>>
>> def formset_factory(form, formset=BaseFormSet, extra=1, can_order=False,
>>                     can_delete=False, max_num=None, validate_max=False,
>>                     min_num=None, validate_min=False):
>>     """Return a FormSet for the given form class."""
>>     if min_num is None:
>>         min_num = DEFAULT_MIN_NUM
>>     if max_num is None:
>>         max_num = DEFAULT_MAX_NUM
>>     # hard limit on forms instantiated, to prevent memory-exhaustion
>> attacks
>>     # limit is simply max_num + DEFAULT_MAX_NUM (which is
>> 2*DEFAULT_MAX_NUM
>>     # if max_num is None in the first place)
>>     absolute_max = max_num + DEFAULT_MAX_NUM
>>     attrs = {'form': form, 'extra': extra,
>>              'can_order': can_order, 'can_delete': can_delete,
>>              'min_num': min_num, 'max_num': max_num,
>>              'absolute_max': absolute_max, 'validate_min': validate_min,
>>              'validate_max': validate_max}
>>     return type(form.__name__ + str('FormSet'), (formset,), attrs)
>>
>>
>>
>> This attribute gets set in several places, but checked only twice in
>> the Django codebase. Unfortunately, I think switching to the proposed
>> inheritance mechanism would make the code worse, not better:
>> `self.can_order` would become `isinstance(self, collections.Ordered)`.
>> The readability of the Django internal code would not be much
>> different, but users would lose consistency of introspectability as
>> the other features like `can_delete` are simple class attributes.
>>
>>
>>
>> On Mon, Nov 9, 2015 at 11:29 AM, Guido van Rossum <guido at python.org>
>> wrote:
>> > Well, that would still defeat the purpose, wouldn't it? The items are no
>> > more ordered than the headers dict itself. Also, items() doesn't return
>> a
>> > sequence -- it's an ItemsView (which inherits from Set) and presumably
>> it's
>> > not Ordered.
>> >
>> > I guess my question is not so much how to prevent getting an exception
>> --
>> > I'm trying to tease out what the right order for the headers would be.
>> Or
>> > perhaps I'm just trying to understand what the code is doing (the
>> snippet
>> > shown mostly looks like bad code to me).
>> >
>> > On Mon, Nov 9, 2015 at 12:03 AM, Ram Rachum <ram at rachum.com> wrote:
>> >>
>> >> I'm not Andrew, but I'm guessing simply writing
>> >> `OrderedDict(headers.items())`.
>> >>
>> >> On Mon, Nov 9, 2015 at 6:28 AM, Guido van Rossum <guido at python.org>
>> wrote:
>> >>>
>> >>> So if OrderedDict had always rejected construction from a dict, how
>> would
>> >>> you have written this?
>> >>>
>> >>>
>> >>> On Sunday, November 8, 2015, Andrew Barnert via Python-ideas
>> >>> <python-ideas at python.org> wrote:
>> >>>>
>> >>>> On Nov 8, 2015, at 14:10, Serhiy Storchaka <storchaka at gmail.com>
>> wrote:
>> >>>> >
>> >>>> >> On 08.11.15 23:12, Sjoerd Job Postmus wrote:
>> >>>> >> On 8 Nov 2015, at 20:06, Amir Rachum <amir at rachum.com
>> >>>> >> <mailto:amir at rachum.com>> wrote:
>> >>>> >>> As part of BasicStruct I intend to allow the use of mapping
>> types as
>> >>>> >>> __slots__, with the semantics of default values..
>> >>>> >>> So it'll look something like this:
>> >>>> >>>
>> >>>> >>>    class Point(BasicStruct):
>> >>>> >>>        __slots__ = {'x': 5, 'y': 7}
>> >>>> >>
>> >>>> >> So instead they'll write
>> >>>> >>     __slots__ = OrderedDict({'x': 5, 'y': 7})
>> >>>> >> Causing the same issues?
>> >>>> >
>> >>>> > Perhaps OrderedDict should reject unordered sources. Hey, here is
>> yet
>> >>>> > one use case!
>> >>>>
>> >>>> I've maintained code that does this:
>> >>>>
>> >>>>     self.headers = OrderedDict(headers)
>> >>>>     self.origheaders = len(headers)
>> >>>>
>> >>>> … so it can later do this:
>> >>>>
>> >>>>     altheaders = list(self.headers.items())[self.origheaders:]
>> >>>>
>> >>>> Not a great design, but one that exists in the wild, and would be
>> broken
>> >>>> by OrderedDict not allowing a dict as an argument.
>> >>>>
>> >>>> Also, this wouldn't allow creating an OrderedDict from an empty dict
>> >>>> (which seems far less stupid, but I didn't lead with it because I
>> can't
>> >>>> remember seeing it in real code).
>> >>>>
>> >>>> _______________________________________________
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>> >>>
>> >>>
>> >>>
>> >>> --
>> >>> --Guido (mobile)
>> >>>
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>> >>
>> >>
>> >
>> >
>> >
>> > --
>> > --Guido van Rossum (python.org/~guido)
>> >
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>>
>
>
>
> --
> --Guido van Rossum (python.org/~guido)
>
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