isinstance(constantdict(), dict) returns True

[matthiasdiener]

Same for:

• isinstance(constantdict(), MutableMapping)
• issubclass(constantdict, dict)
• issubclass(constantdict, MutableMapping)

https://stackoverflow.com/questions/57982946/how-to-register-implementation-of-abc-mutablemapping-as-a-dict-subclass

[matthiasdiener]

Sample error from loopy (via inducer/loopy#884):

  File "/Users/mdiener/Work/emirge/miniforge3/envs/ceesd/lib/python3.11/site-packages/pytools/persistent_dict.py", line 708, in store
    v = pickle.dumps((key, value))
        ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mdiener/Work/emirge/loopy/loopy/translation_unit.py", line 469, in __getstate__
    return asdict(self)
           ^^^^^^^^^^^^
  File "/Users/mdiener/Work/emirge/miniforge3/envs/ceesd/lib/python3.11/dataclasses.py", line 1286, in asdict
    return _asdict_inner(obj, dict_factory)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mdiener/Work/emirge/miniforge3/envs/ceesd/lib/python3.11/dataclasses.py", line 1293, in _asdict_inner
    value = _asdict_inner(getattr(obj, f.name), dict_factory)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mdiener/Work/emirge/miniforge3/envs/ceesd/lib/python3.11/dataclasses.py", line 1323, in _asdict_inner
    return type(obj)((_asdict_inner(k, dict_factory),
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mdiener/Work/emirge/miniforge3/envs/ceesd/lib/python3.11/dataclasses.py", line 1324, in <genexpr>
    _asdict_inner(v, dict_factory))
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mdiener/Work/emirge/miniforge3/envs/ceesd/lib/python3.11/dataclasses.py", line 1293, in _asdict_inner
    value = _asdict_inner(getattr(obj, f.name), dict_factory)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mdiener/Work/emirge/miniforge3/envs/ceesd/lib/python3.11/dataclasses.py", line 1293, in _asdict_inner
    value = _asdict_inner(getattr(obj, f.name), dict_factory)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mdiener/Work/emirge/miniforge3/envs/ceesd/lib/python3.11/dataclasses.py", line 1321, in _asdict_inner
    return type(obj)(_asdict_inner(v, dict_factory) for v in obj)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mdiener/Work/emirge/miniforge3/envs/ceesd/lib/python3.11/dataclasses.py", line 1321, in <genexpr>
    return type(obj)(_asdict_inner(v, dict_factory) for v in obj)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mdiener/Work/emirge/miniforge3/envs/ceesd/lib/python3.11/dataclasses.py", line 1323, in _asdict_inner
    return type(obj)((_asdict_inner(k, dict_factory),
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: unhashable type: 'dict'

[matthiasdiener]

An approach similar to https://github.com/inducer/arraycontext/blob/029026cca4655be0314751cb2a2abfc26bc3732b/arraycontext/container/arithmetic.py#L126-L132 may not work:

>>> class CDMetaClass(type):
...     def __instancecheck__(cls, instance) -> bool:
...         print(" -- instancecheck", cls, instance, cls == type(instance))
...         return cls == type(instance)
...
>>> class constantdict(dict, metaclass=CDMetaClass):
...     pass
...
>>> obj1 = constantdict({1:2})
>>>
>>> print(isinstance(obj1, dict))
True  # should be False, but __instancecheck__ isn't even called
>>> print(isinstance(obj1, constantdict))
True  # correct result, but __instancecheck__ was also not called

cc @inducer

[inducer]

Ugh, interesting. The root base class sets the metaclass. Recreating a custom dict with the desired metaclass also doesn't work:

class CDMetaClass(type):
    def __instancecheck__(cls, instance) -> bool:
        print(" -- instancecheck", cls, instance, cls is type(instance))
        return cls is type(instance)
basedict = CDMetaClass('basedict', dict.__bases__, dict(vars(dict)))
class constantdict(basedict):
    pass
obj1 = constantdict({"a": 1, "b": 2})
print(isinstance(obj1, dict))
print(isinstance(obj1, constantdict))

Multiple inheritance also can't change metaclasses:

class CDMetaClass(type):
    def __instancecheck__(cls, instance) -> bool:
        print(" -- instancecheck", cls, instance, cls is type(instance))
        return cls is type(instance)
class MetaBase(metaclass=CDMetaClass):
    pass
class constantdict(dict, MetaBase):
    pass
class constantdict2(MetaBase, dict): # also nope
    pass
obj1 = constantdict({"a": 1, "b": 2})
print(isinstance(obj1, dict))
print(isinstance(obj1, constantdict))