Python isinstance() Function

The isinstance() function in Python checks whether a variable is an instance of a specific class or a tuple of classes. It's a safe and recommended way to validate data types.

Syntax

isinstance(object, classinfo)

Parameters:

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

Returns:

• True if object is an instance (or subclass instance) of classinfo.
• False otherwise.

Example 1: Check if a variable is an integer

x = 5
print(isinstance(x, int))

True

Example 2: Check if a string is a list (it isn’t)

name = "Alice"
print(isinstance(name, list))

False

Example 3: Check against multiple types

value = 3.14
print(isinstance(value, (int, float)))

True

Here, value is checked against both int and float. The function returns True if it matches any one.

Use Case: Why Use isinstance()?

• To validate function arguments
• To avoid type errors in logic
• To write type-safe code, especially in large projects
• To apply different logic based on variable type (e.g., float vs int)

Example 4: Type checking in a function

def square(x):
    if isinstance(x, (int, float)):
        return x * x
    else:
        return "Invalid input"

print(square(4))      # 16
print(square("four")) # Invalid input

16
Invalid input

Common Mistakes

• Passing a string for classinfo like "int" (should be int without quotes)
• Confusing type() with isinstance(): isinstance() supports inheritance, type() does not

Interview Tip

isinstance() is often used in questions involving type safety or when a function needs to behave differently based on data types. It’s also commonly used in object-oriented questions.

Summary

• isinstance(obj, type) returns True if obj is of the given type or its subclass.
• Use a tuple to check against multiple types.
• Safer than using type() when inheritance is involved.

Practice Problem

Write a function that accepts a value and returns:

• "Integer" if it’s an integer
• "Float" if it’s a float
• "Other" otherwise

def identify_type(x):
    if isinstance(x, int):
        return "Integer"
    elif isinstance(x, float):
        return "Float"
    else:
        return "Other"

print(identify_type(42))
print(identify_type(3.14))
print(identify_type("hello"))

Integer
Float
Other