How to Use isinstance() in Python to Validate Data Types With Examples

 

What is isinstance() in Python?

isinstance() is a built-in Python function used to check if a variable is of a specific type or class. It's often used for data validation, type checking, and writing safer, more readable code.

 Syntax

isinstance(object, classinfo)

• object: the variable you want to check
• classinfo: a type or a tuple of types (e.g., int, str, (int, float))

Returns:

• True if the object matches the type(s)
• False otherwise

 Example 1: Check for Integer

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

Output:

x is an integer

 Example 2: Multiple Types

x = 3.14
if isinstance(x, (int, float)):
    print("x is a number")

Output:

x is a number

 Example 3: Validate User Input

user_input = "123"

if isinstance(user_input, str):
    try:
        num = int(user_input)
        print("Converted to integer:", num)
    except ValueError:
        print("Not a valid number string")

 Use Case Examples

Use Case	Code Example
Check if a value is a list	isinstance(data, list)
Check if value is a number	isinstance(x, (int, float))
Avoid AttributeError on wrong type	Use isinstance(obj, str) before .upper()
Validate function arguments	Combine with raise TypeError(...) if invalid

 Common Mistakes

 Using type() for validation in the wrong way:

type(5) == int  # Works, but less flexible

 Better:

isinstance(5, int)

isinstance() supports inheritance, while type() does not.

 Advanced Tip: Custom Classes

class Animal:
    pass

class Dog(Animal):
    pass

d = Dog()
print(isinstance(d, Animal))  #  True, because Dog is a subclass of Animal

 Summary

Task	Recommended Code
Check for a specific type	isinstance(x, int)
Check for multiple types	isinstance(x, (int, float))
Check before method call	if isinstance(s, str): s.upper()
Works with inheritance	Yes