issubclass

issubclass is a built-in Python function that determines whether one class is a subclass of another. It is used to query the inheritance hierarchy at runtime and can reflect direct and indirect inheritance, including cases involving multiple inheritance and virtual subclassing via abstract base classes.

The signature is issubclass(cls, class_or_tuple). The first argument, cls, must be a class object. The second argument, class_or_tuple, may be a class object or a tuple of class objects. The function returns True if cls is a subclass of any of the classes in class_or_tuple, or if cls is the same class as one of them. If class_or_tuple is a tuple, the check is performed against each element; any match yields True, otherwise False. If the relationship does not hold, False is returned.

Both arguments must be class objects; passing a non-class value for either argument raises a TypeError. The check is implemented via the class’s method resolution order (MRO) and respects Python’s rules for multiple inheritance and virtual subclassing supported by abstract base classes.

In practice, issubclass is often used to guard behavior that depends on type hierarchy, in contrast to isinstance, which tests instances rather than classes. Examples: class Animal: pass class Dog(Animal): pass issubclass(Dog, Animal) -> True; issubclass(Dog, (Animal, str)) -> True; issubclass(Animal, Dog) -> False. Note that issubclass(int, int) is True.