How to Validate Input and Raise TypeError in Python

Define a function that checks its argument type and raises a TypeError early with a clear message when given a non-number.

Easy Python 3.6+ Aug 9, 2026 Errors & debugging 14 views 0 copies

type checking validation typeerror error handling

Python code

11 lines

Python 3.6+

def validate_number(value):
    if not isinstance(value, (int, float)):
        raise TypeError(f"Expected a number, got {type(value).__name__}")
    return value * 2

if __name__ == "__main__":
    try:
        print(validate_number(5))
        print(validate_number("hello"))
    except TypeError as e:
        print(f"Error: {e}")

Output

stdout

10
Error: Expected a number, got str

How it works

The isinstance check ensures only int or float values are accepted; otherwise a TypeError is raised with a descriptive message that includes the actual type name. By raising the error early, you avoid confusing downstream failures. The try/except block in the main guard demonstrates how to catch and handle the error gracefully.

Common mistakes

Using `type(value) == int` which fails for subclasses or boolean values
Raising a generic `ValueError` instead of the specific `TypeError`
Checking `isinstance(value, (int, float))` but forgetting `bool` is a subclass of `int`
Not including the actual type in the error message

Variations

Use Python 3.10+ with `match value:` to validate type patterns
Use a type annotation like `def validate_number(value: int) -> int:` combined with a runtime check

Real-world use cases

Validating user input before performing arithmetic in a CLI calculator script.
Checking configuration parameters at service startup to fail fast with clear errors.
Ensuring numeric fields in API request handlers are the correct type before processing.

How to Validate Input and Raise TypeError in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Output

How it works

Common mistakes

Variations

Real-world use cases

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