isinstance()

The built-in isinstance() function checks if an object is an instance of a specified class or a subclass thereof, returning a Boolean value. It’s a versatile tool for explicit type checking in Python, as it considers subclass relationships:

Python 
>>> isinstance(42, int)
True

isinstance() Signature

Python Syntax 
isinstance(object, classinfo)

Arguments

Argument Description
object The object to check.
classinfo A class, tuple of classes, or union type to check against.

Return Value

  • Returns True if the object is an instance of classinfo or a subclass thereof.
  • Returns False if the object isn’t an instance of classinfo.

isinstance() Examples

With a number and class as arguments:

Python 
>>> isinstance(3.14, float)
True
>>> isinstance(3.14, int)
False

With a number and a tuple of classes as arguments:

Python 
>>> isinstance(42, (int, float))
True

With a union type as an argument:

Python 
>>> isinstance(42.0, int | float)
True

isinstance() Common Use Cases

The most common use cases for the isinstance() function include:

  • Explicitly checking data types in conditional statements
  • Ensuring input data conforms to expected data types
  • Validating arguments in functions and methods

isinstance() Real-World Example

Say you’re implementing a function that processes different types of numerical data. You can use isinstance() to ensure the input is of a valid type:

Python 
>>> def process_number(number):
...     if isinstance(number, (int, float)):
...         return number ** 2
...     else:
...         raise TypeError("the input must be an int or float")

>>> process_number(4)
16
>>> process_number(4.5)
20.25
>>> process_number("4")
Traceback (most recent call last):
    ...
TypeError: the input must be an int or float

In this example, isinstance() ensures that the function processes only integers and floats, and raises a TypeError otherwise.

What Does isinstance() Do in Python?

Tutorial

What Does isinstance() Do in Python?

Learn what isinstance() does in Python and how to use this built-in function to check an object's type. Discover its practical uses along with key limitations.

intermediate python

For additional information on related topics, take a look at the following resources:

By Leodanis Pozo Ramos • Updated July 22, 2025 • Reviewed by Dan Bader
Python’s Built-in Functions Share Feedback