isinstance

The isinstance function checks whether an object is an instance of a class or of a subclass thereof:

class A: pass
class B(A): pass
b = B()
isinstance(b, B) # True
isinstance(b, A) # True
isinstance(b, object) # True
isinstance(b, str) # False
isinstance(str, type) # True

Type-checkers understand isinstance checks and use them to refine the type:

a: object
reveal_type(a)
# ^ Revealed type is "builtins.object"
if isinstance(a, str):
    reveal_type(a)
    # ^ Revealed type is "builtins.str"

One more cool thing about isinstance is that you can pass in it a tuple of types to check if the object is an instance of any of them:

isinstance(1, (str, int)) # True