[issue43829] MappingProxyType cannot hash a hashable underlying mapping

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

But there is an issue with comparison implementation in MappingProxyType (see 
issue43838). 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).

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[issue43829] MappingProxyType cannot hash a hashable underlying mapping

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

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.

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[issue43829] MappingProxyType cannot hash a hashable underlying mapping

[Raymond Hettinger]

Raymond Hettinger  added the comment:

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.

--
nosy: +rhettinger, serhiy.storchaka
priority: normal -> low
versions: +Python 3.10 -Python 3.9

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[issue43829] MappingProxyType cannot hash a hashable underlying mapping

[Andy Maier]

New submission from Andy Maier :

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 
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.

--
components: Library (Lib)
messages: 390936
nosy: andymaier
priority: normal
severity: normal
status: open
title: MappingProxyType cannot hash a hashable underlying mapping
type: behavior
versions: Python 3.9

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