python - Why is NotImplementedType not a type subclass? -

i noticed strange when inspecting built-in type notimplementedtype.

>>> types import notimplementedtype >>> issubclass(notimplementedtype, type) false >>> type(notimplementedtype) <type 'type'> 

how can these 2 things true? how can notimplementedtype not subclass of type yet derived type?

classes not subclass of type, including types.notimplementedtype. type metaclass of classes.

for example, custom classes , built-in types not subclasses of type either:

>>> class foo: pass ... >>> issubclass(foo, type) false >>> issubclass(int, type) false 

only other metaclasses subclasses of type; abcmeta metaclass:

>>> abc import abcmeta >>> issubclass(abcmeta, type) true 

this analogous intances , classes; instances not subclasses of class; use isinstance():

>>> issubclass(foo(), foo) traceback (most recent call last):   file "<stdin>", line 1, in <module> typeerror: issubclass() arg 1 must class >>> isinstance(foo(), foo) true >>> import types >>> isinstance(types.notimplementedtype, type) true 

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