Dict views should be introspectable

[rhettinger]

BPO	40890
Nosy	@rhettinger, @jab, @methane, @serhiy-storchaka, @remilapeyre, @sweeneyde
PRs	bpo-40890: Add mapping attribute to dict views #20691 bpo-40890: Add mapping property to dict views #20749 bpo-40890: Fix the signature of the dictview.mapping getter #20873 bpo-40890: Fix compiler warning in dictobject.c #20876 Add MappingView.mapping for parity with dictview #28892

^{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 = None
closed_at = <Date 2020-06-15.05:55:24.199>
created_at = <Date 2020-06-06.17:21:19.846>
labels = ['interpreter-core', 'type-bug', '3.10']
title = 'Dict views should be introspectable'
updated_at = <Date 2021-10-11.23:25:28.222>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2021-10-11.23:25:28.222>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2020-06-15.05:55:24.199>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2020-06-06.17:21:19.846>
creator = 'rhettinger'
dependencies = []
files = []
hgrepos = []
issue_num = 40890
keywords = ['patch']
message_count = 19.0
messages = ['370841', '370875', '370876', '370877', '370882', '370884', '370885', '370892', '370893', '370914', '371070', '371072', '371081', '371121', '371155', '371337', '371398', '371506', '371517']
nosy_count = 6.0
nosy_names = ['rhettinger', 'jab', 'methane', 'serhiy.storchaka', 'remi.lapeyre', 'Dennis Sweeney']
pr_nums = ['20691', '20749', '20873', '20876', '28892']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40890'
versions = ['Python 3.10']

[rhettinger]

Dict views wrap an underlying mapping but don't expose that mapping as an attribute.

Traditionally, we do expose wrapped objects: property() exposes fget, partial() exposes func, bound methods expose __func__, ChainMap() exposes maps, etc.

Exposing this attribute would help with introspection, making it possible to write efficient functions that operate on dict views.

[sweeneyde]

I'd be happy to write a PR.

Method names could be "mapping", "target", "target_mapping", "target_dict", "referent_dict", etc.

I like the choice of "target_mapping":

    d = dict()
    assert d is d.keys().target_mapping
    assert d is d.values().target_mapping
    assert d is d.items().target_mapping

[rhettinger]

Just "mapping" would suffice. That is variable name used in the ABCs, the name defined in the glossary, and the variable name used in ChainMap (in plural form).

[sweeneyde]

I think this will also require typing.MappingProxyType to change a bit, since it would make a proxy's underlying dict accessible:

    >>> d = dict()
    >>> proxy = MappingProxyType(d)
    >>> type(proxy.items()) is type(d.items())  # should be False
    True
    >>> proxy.items().mapping is d  # should be False
    ???

[sweeneyde]

Indeed, with PR 20691 applied, the following crashes:

>> vars(str).items().mapping.clear()
>> "uh oh"

[sweeneyde]

Would the best way to address this be adding new KeysProxy, ValuesProxy, and ItemsProxy types?

[serhiy-storchaka]

I am not sure that it is good idea to expose internal details (do you want to expose also internal details of iterators?), but if expose an underlying dict of a dict view, I think it should be either a copy or a read-only proxy. It should be enough to help with introspection, but will not open the hole in integrity.