Description

assignee = None
closed_at = None
created_at = <Date 2021-04-13.08:25:32.893>
labels = ['type-bug', 'library', '3.10']
title = 'MappingProxyType cannot hash a hashable underlying mapping'
updated_at = <Date 2021-04-14.07:46:45.086>
user = 'https://github.com/andy-maier'

activity = <Date 2021-04-14.07:46:45.086>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-04-13.08:25:32.893>
creator = 'andymaier'
dependencies = []
files = []
hgrepos = []
issue_num = 43829
keywords = []
message_count = 4.0
messages = ['390936', '391028', '391038', '391041']
nosy_count = 3.0
nosy_names = ['rhettinger', 'serhiy.storchaka', 'andymaier']
pr_nums = []
priority = 'low'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43829'
versions = ['Python 3.10']

Activity

1. andy-maier commented on Apr 13, 2021

   

   andy-maiermannequin

   on Apr 13, 2021

   MannequinAuthor

   More actions

   Objects of MappingProxyType do expose a __hash__() method, but if the underlying mapping is hashable, it still does not support hashing it.

   Example:

   Content of mp_hash.py:

   ------
   #!/usr/bin/env python

   from nocasedict import NocaseDict, HashableMixin
   from types import MappingProxyType
   
   class HashableDict(HashableMixin, NocaseDict):
       """A hashable dictionary"""
       pass
   
   hd = HashableDict({'a': 1, 'b': 2})
   print("hash(hd): {}".format(hash(hd)))
   
   mp = MappingProxyType(hd)
   print("hash(mp): {}".format(hash(mp)))

   ---

   Running the mp_hash.py script:

   hash(hd): 3709951335832776636
   Traceback (most recent call last):
     File "/Users/maiera/Projects/Python/cpython/issues/mappingproxy/./mp_hash.py", line 14, in <module>
       print("hash(mp): {}".format(hash(mp)))
   TypeError: unhashable type: 'mappingproxy'

   There are use cases where a function wants to return an immutable view on an internal dictionary, and the caller of the function should be able to use the returned object like a dictionary, except that it is read-only.

   Note there is https://bugs.python.org/issue31209 on the inability to pickle MappingProxyType objects which was closed without adding the capability. That would fall under the same argument.

   Reacted by Václav Brožík and Jan-Hein Bührman
2. andy-maier

   added

   3.9 (EOL)end of lifeend of life

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Apr 13, 2021
3. rhettinger commented on Apr 14, 2021

   

   rhettinger

   on Apr 14, 2021

   Contributor

   More actions

   Serhiy, what do you think? Would it make sense to pass through the underlying hash? I don't think there is much use for this but don't see any particular reason to block it.
4. rhettinger

   added

   3.10 (EOL)end of lifeend of life

   and removed

   3.9 (EOL)end of lifeend of life

   on Apr 14, 2021
5. serhiy-storchaka commented on Apr 14, 2021

   

   serhiy-storchaka

   on Apr 14, 2021

   Member

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   Perhaps MappingProxyType is unhashable by accident. It implements __eq__, and it makes it unhashable by default. And nobody made request for this feature before. I think that implementing __hash__ would not make anything wrong.
6. serhiy-storchaka commented on Apr 14, 2021

   

   serhiy-storchaka

   on Apr 14, 2021

   Member

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   But there is an issue with comparison implementation in MappingProxyType (see bpo-43838). Resolving that issue can affect the decision about hashability.

   There should be always true the following predicate: if x == y then hash(x) == hash(y).
7. ezio-melotti

   transferred this issue fromon Apr 10, 2022
8. vbrozik commented on Jun 21, 2022

   

   vbrozik

   on Jun 21, 2022

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   The issue #88004 (bpo-43838) was already closed with reasoning:

   ... the reason we proxy in the first place is not absolute safety but to make sure
   people don’t accidentally update the dict when they intend to update the
   class ...

   In general, it is hard to completely wall-off instances against deliberate efforts to pry them open

   I do not se a reason why not allow the hashing. Recently I needed to collect a set of small unique dict objects. It is a pity that I could not utilize MappingProxyType directly.
9. serhiy-storchaka

   added a commit that references this issue on Jun 25, 2022

   pythongh-87995: Make MappingProxyType hashable

   7afb2d2
10. ambv

    added a commit that references this issue on Jun 28, 2022

    gh-87995: Make MappingProxyType hashable (GH-94252)

    efdc9d6
11. ambv commented on Jun 28, 2022

    

    ambv

    on Jun 28, 2022

    Contributor

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    This is now fixed. Thanks, Serhiy! ✨ 🍰 ✨
12. ambv

    closed this as completedon Jun 28, 2022
13. gvanrossum

    added a commit that references this issue on Jun 30, 2022

    pythongh-87995: Make MappingProxyType hashable (pythonGH-94252)

    98082d0
14. matanox commented on Dec 6, 2023

    

    matanox

    on Dec 6, 2023

    More actions

    Does this mean that this should no longer issue an "unhashable type" TypeError?

    hash(MappingProxyType({3:2, 4:3}))
    

    Or is this feature indeed coming as of python 3.12?

Issue actions