New submission from Andy Maier <andreas.r.ma...@gmx.de>:
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.
----------
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
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue43829>
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