Python issubclass() Function

The issubclass() function checks if a class is a subclass of another class or a tuple of classes. It is used in object-oriented programming to verify inheritance relationships.

Syntax

issubclass(class, classinfo)

Parameters:

• class – The class you want to check.
• classinfo – A class or a tuple of classes to check against.

Returns:

• True if class is a subclass (direct or indirect) of classinfo.
• False otherwise.

Example 1: Basic Usage

class Animal:
    pass

class Dog(Animal):
    pass

print(issubclass(Dog, Animal))

True

Example 2: Indirect Subclass

class Animal:
    pass

class Mammal(Animal):
    pass

class Cat(Mammal):
    pass

print(issubclass(Cat, Animal))

True

Why? Because Cat → Mammal → Animal. So, Cat is an indirect subclass of Animal.

Example 3: Using Tuple

class Vehicle: pass
class Car(Vehicle): pass

print(issubclass(Car, (Vehicle, Animal)))

True

Returns True if Car is a subclass of any class in the tuple.

Example 4: Invalid Check

print(issubclass(5, int))

TypeError: issubclass() arg 1 must be a class

Note: You must pass a class as the first argument, not an instance.

Use Cases

• Validating object types in class hierarchies
• Conditional behavior in frameworks using inheritance
• Metaprogramming and decorators

Common Mistakes

• ❌ Passing an instance instead of a class: issubclass(dog, Animal) ❌
• Instead, use: isinstance(dog, Animal) to check instances

Interview Tip

issubclass() is often tested with isinstance(). Know the difference: one checks classes, the other checks instances.

Summary

• issubclass(A, B) returns True if A is a subclass of B
• Supports tuples of classes
• Raises TypeError if the first argument is not a class

Practice Problem

Create a class hierarchy with Shape, Polygon, and Triangle. Use issubclass() to check if Triangle is a subclass of Shape.

class Shape: pass
class Polygon(Shape): pass
class Triangle(Polygon): pass

print(issubclass(Triangle, Shape))

Expected Output:

True