Message 390936 - Python tracker (original) (raw)

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.