Testing custom exceptions in PyTest

Introduction

We test custom exceptions to make sure our program handles errors the way we expect. This helps catch problems early and keeps the program stable.

When you want to check if your code raises a specific error for bad input.

When you create your own error types to explain problems clearly.

When you want to confirm that error messages are correct and helpful.

When you want to prevent your program from crashing unexpectedly.

When you want to document how your code behaves in error situations.

Syntax

PyTest

import pytest

with pytest.raises(ExpectedException):
    function_that_raises()

Use pytest.raises() as a context manager to check for exceptions.

Replace ExpectedException with your custom exception class.

Examples

This example tests that bad_func() raises the custom MyError.

PyTest

import pytest

class MyError(Exception):
    pass

def bad_func():
    raise MyError("Oops")

with pytest.raises(MyError):
    bad_func()

This example also checks the error message text.

PyTest

import pytest

class CustomError(Exception):
    pass

def check_value(x):
    if x < 0:
        raise CustomError("Negative value")

with pytest.raises(CustomError) as exc_info:
    check_value(-1)

assert str(exc_info.value) == "Negative value"

Sample Program

This test checks that set_age raises InvalidAgeError for invalid ages and returns the age for valid input.

PyTest

import pytest

class InvalidAgeError(Exception):
    pass

def set_age(age):
    if age < 0 or age > 120:
        raise InvalidAgeError("Age must be between 0 and 120")
    return age

def test_set_age():
    with pytest.raises(InvalidAgeError) as e:
        set_age(-5)
    assert str(e.value) == "Age must be between 0 and 120"

    with pytest.raises(InvalidAgeError):
        set_age(150)

    assert set_age(30) == 30

Important Notes

Always test both that the exception is raised and that the message is correct.

Use clear and descriptive names for your custom exceptions.

pytest shows a clear report when tests pass or fail, making debugging easier.

Summary

Testing custom exceptions ensures your code handles errors properly.

Use pytest.raises() to check for exceptions in tests.

Check both the type of exception and its message for full coverage.