types.MappingProxyType and collections.defaultdict

[guoci]

BPO	44867
Nosy	@rhettinger, @guoci

^{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 2021-08-08.20:19:44.313>
created_at = <Date 2021-08-08.19:16:15.848>
labels = ['type-bug', '3.8', '3.9', '3.10', '3.11', 'invalid', '3.7', 'library']
title = 'types.MappingProxyType and collections.defaultdict'
updated_at = <Date 2021-08-08.20:19:44.310>
user = 'https://github.com/guoci'

bugs.python.org fields:

activity = <Date 2021-08-08.20:19:44.310>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2021-08-08.20:19:44.313>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2021-08-08.19:16:15.848>
creator = 'guoci'
dependencies = []
files = []
hgrepos = []
issue_num = 44867
keywords = []
message_count = 2.0
messages = ['399234', '399238']
nosy_count = 2.0
nosy_names = ['rhettinger', 'guoci']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44867'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10', 'Python 3.11']

[guoci]

types.MappingProxyType is documented as 'Read-only proxy of a mapping'.
But if used with a collections.defaultdict mapping, it can modify the underlying mapping.

import collections, types
dd = collections.defaultdict(set)
mpt = types.MappingProxyType(dd)
mpt['__getitem__'] # key inserted
mpt.get('get') # key not inserted
print(dd.items()) # dict_items([('__getitem__', set())])

[rhettinger]

types.MappingProxyType is documented as
'Read-only proxy of a mapping'. But if used with a
collections.defaultdict mapping, it can modify
the underlying mapping.

Technically, the underlying mapping is modifying itself. That is allowed.

Also, there isn't really anything that the mapping proxy can do about it. All MappingProxy can do it forward calls to methods that are usually non-mutating.