How to Use isinstance Function in Python: Simple Guide

Use the isinstance(object, classinfo) function in Python to check if an object is an instance of a class or a tuple of classes. It returns True if the object matches the type, otherwise False. This helps you safely test an object's type before using it.

Syntax

The isinstance function takes two arguments: the object you want to check, and the class or tuple of classes you want to check against.

• object: The variable or value to test.
• classinfo: A class, type, or a tuple of classes/types.

It returns True if the object is an instance of the classinfo or any subclass, otherwise False.

python

isinstance(object, classinfo)

Example

This example shows how to check if a variable is an integer or a string using isinstance. It prints messages based on the type.

python

x = 10
if isinstance(x, int):
    print("x is an integer")

name = "Alice"
if isinstance(name, str):
    print("name is a string")

# Check multiple types
value = 3.14
if isinstance(value, (int, float)):
    print("value is a number")

Output

x is an integer
name is a string
value is a number

Common Pitfalls

Some common mistakes when using isinstance include:

• Using type() instead of isinstance(), which does not consider inheritance.
• Passing a single class inside a tuple by mistake, like (int) instead of (int,).
• Checking against mutable types incorrectly.

Always use isinstance for type checks to support inheritance and multiple types.

python

class Animal:
    pass

class Dog(Animal):
    pass

d = Dog()

# Wrong: type() does not consider inheritance
print(type(d) == Animal)  # False

# Right: isinstance() considers inheritance
print(isinstance(d, Animal))  # True

# Wrong tuple with one element (missing comma)
print(isinstance(d, (Dog)))  # False

# Correct tuple with one element
print(isinstance(d, (Dog,)))  # True

Output

False
True
False
True

Quick Reference

Remember these tips when using isinstance:

• Use it to check if an object matches a type or any subclass.
• Pass a tuple to check multiple types at once.
• It returns True or False.
• Supports inheritance, unlike type().

Key Takeaways

Use isinstance(object, classinfo) to check an object's type safely.

It returns True if the object is an instance of the class or any subclass.

Pass a tuple of classes to check multiple types at once.

Avoid using type() for type checks when inheritance matters.

Remember to use a comma for single-element tuples in classinfo.