The built-in isinstance() reads an instance's __class__ with a lookup that
suppresses AttributeError and falls back to the object's type, so
isinstance(obj, int) returns False for an object whose __class__ access
raises. ABCMeta.__instancecheck__ read __class__ directly instead, so
isinstance(obj, Mapping) leaked that AttributeError.
Fall back to type(instance) when __class__ is unavailable, in both the C
and the pure-Python implementations, so the abstract base classes behave
like the built-in isinstance(). Such objects are unusual, but they do turn
up in the wild (for example some Qt widgets).
Vyron Vasileiadis