dict_values isn't considered a Collection nor a Container

[yahya-abou-imran]

BPO	32467
Nosy	@rhettinger, @yahya-abou-imran
PRs	bpo-32467: Let collections.abc.ValuesView inherit from Collection #5152

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/rhettinger'
closed_at = <Date 2018-01-12.05:54:15.840>
created_at = <Date 2017-12-31.15:36:24.073>
labels = ['3.7', 'type-bug', 'library']
title = "dict_values isn't considered a Collection nor a Container"
updated_at = <Date 2018-01-12.05:54:15.840>
user = 'https://github.com/yahya-abou-imran'

bugs.python.org fields:

activity = <Date 2018-01-12.05:54:15.840>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2018-01-12.05:54:15.840>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2017-12-31.15:36:24.073>
creator = 'yahya-abou-imran'
dependencies = []
files = []
hgrepos = []
issue_num = 32467
keywords = ['patch']
message_count = 4.0
messages = ['309289', '309795', '309813', '309837']
nosy_count = 2.0
nosy_names = ['rhettinger', 'yahya-abou-imran']
pr_nums = ['5152']
priority = 'low'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32467'
versions = ['Python 3.7']

[yahya-abou-imran]

a dict_values instance behaves like a Collection (a Sized Iterable Container):

>>> values = {1: 'one', 2: 'two'}.values()
>>> 'two' in values
True
>>> 'three' in values
False
>>> for value in values: print(value)
one
two
>>> len(values)
2

But...

>>> isinstance(values, abc.Collection)
False
>>> isinstance(values, abc.Container)
False

The reason is the lack of __contains__():

>>> '__contains__' in dir(values)
False

The 'in' operator works above because it uses __iter__() to check the presence of the value.

I think it's inconsistent with the fact that dict_values is in the registry of ValuesView witch passes the test:

>>> issubclass(abc.ValuesView, abc.Collection)
True

A class passed in the registry of ValuesView should guarantee this behaviour (witch is the case here by the way, but informally).

I can think about several solutions for this:

1. add a __contains__() to the dict_values class;
2. if an ABC is considered a subclass of another ABC, all the classes in the registry of the first (and recursively all of their subclasses) should be too;
3. pass dict_values in the registry of Container or Collection;
4. make ValuesView inherit from Container or Collection.

IMHO:

1 is out.
2 makes sense, but may be difficult to implement since it implies that a class has to know all the registries it's been passed in.
3 and 4 are by far the easiest ways to do it.

I think the inheritance from Collection is the best solution, because KeysView and ItemsView are already Collections by inheriting from Set witch inherits from Collection. The only fondamental difference between the three views (in terms of protocol and API), it's that ValuesView is not a Set.

[rhettinger]

It would be okay to let ValuesView inherit from Collection.

Am marking this a 3.7 only because it isn't important to anyone's actual code and there is no known use case. Technically, it isn't even a bug. The __contains__ method is supported implicitly (as it is for many classes that just define __iter__), so there is no requirement that this be recognized as a Collection. Also, the existing behavior was explicitly tested (see line 848 in Lib/test/test_collections).

I'm going forward with this because it does offer a sense of neatness (in comparison to KeysView and ItemsView) and it might avoid a pedantic StackOverflow question somewhere down the line.

[yahya-abou-imran]

Perfect for me.
I was about to submit about the same patch as yours after your acceptation; but it's better that way, it's your module!
Thanks Raymond.

[rhettinger]

New changeset 02556fb by Raymond Hettinger in branch 'master':
bpo-32467: Let collections.abc.ValuesView inherit from Collection (bpo-5152)
02556fb