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The spec calls out that you can create subclasses of ‘typing.Any’. Part of description confused me at first:
This can be useful for avoiding type checker errors with classes that can duck type anywhere or are highly dynamic.
I first read this to mean that a subclass of ‘Any’ should be treated as another way to spell the Any type. That would lead to:
Every type is assignable to [each subclass of ‘Any’], and [each subclass of ‘Any’] is assignable to every type.
The correct reading is to start from first principles. The Any type means “we don't know”, and so a class that inherits from ‘Any’ has a superclass, but we don't know precisely which one.
That means that only half of the above is correct. An instance of a subclass of ‘Any’ is assignable to every type ‘T’, since we can materialize the ‘Any’ superclass to ‘T’, and an instance of a class is assignable to a variable of that class of any of its superclasses.
But the converse is not true. The following does not type-check:
class Sub(Any):
pass
x: Sub = 4
because the following does not either:
class Sub(int):
pass
x: Sub = 4
(You can assign to a superclass-typed variable, but not to a subclass-typed variable. Liskov substitutability. The “assignable from” relation is not symmetric.)