Python isinstance() Function

Usage

The isinstance() function in Python is a built-in function primarily used to check if an object is an instance of a particular class or any class derived from it.

By passing a tuple containing multiple classes or types to the isinstance() function, you can determine if the object is an instance of any of the specified classes or types.

Syntax

isinstance(object, classinfo)

Parameters

Return Value

The isinstance() function returns a boolean value. It returns True if the given object is an instance of the specified class, or any of its subclasses, and False otherwise.

When the classinfo parameter receives a tuple containing multiple classes or types, the isinstance() function returns True if the object is an instance of any class or type within the tuple; otherwise, it returns False.

Examples

Here’s how to use the isinstance() function:

print(isinstance(42, int))          # True (42 is an integer)
print(isinstance("Hello", str))     # True ("Hello" is a string)
print(isinstance([1, 2, 3], list))  # True ([1, 2, 3] is a list)

print(isinstance(15, float))        # False (15 is not a float)
print(isinstance("World", list))    # False ("World" is not a list)

Checking Against a Custom Class

You can even use isinstance() to check if an object is an instance of a custom class.

class Car():
    pass

my_car = Car()
print(isinstance(my_car, Car))    # True

In this example, isinstance() returns True because my_car is an instance of the Car class.

Using a Tuple for classinfo

You can also pass a tuple to classinfo to check if the object is an instance of any of the classes or types in that tuple:

print(isinstance(42, (int, float)))       # True (42 is an integer)
print(isinstance(3.14, (int, float)))     # True (3.14 is a float)

print(isinstance("Hello", (int, float)))  # False ("Hello" is neither an integer no a float)

It’s essentially the same as performing multiple individual isinstance() checks joined by the OR operator. So, isinstance(x, (A, B, ...)) is equivalent to isinstance(x, A) OR isinstance(x, B) OR ....

print(isinstance(42, int) or isinstance(42, float))       # True
print(isinstance(3.14, int) or isinstance(3.14, float))   # True

isinstance() Accounts for Inheritance

The isinstance() function takes inheritance into account. It checks if an object is an instance of a particular class or any of its subclasses. This means if an object belongs to a derived class (a subclass), isinstance() will still recognize it as being an instance of the parent class.

Let’s illustrate this with an example:

class Fruit:
    pass

class Apple(Fruit):
    pass

apple = Apple()

print(isinstance(apple, Apple))   # True (apple is directly an instance of Apple)
print(isinstance(apple, Fruit))   # True (Apple inherits from Fruit)

In this example, isinstance() returns True in both cases because the apple object is an instance of the Apple class, and, importantly, the Apple class inherits from the Fruit class.

Another interesting example is that booleans (True and False) in Python are a subclass of integers. Therefore, a boolean object will return True when checked with isinstance() against both the bool and int types.

print(isinstance(True, bool))   # True
print(isinstance(True, int))    # True