Asserting exceptions (pytest.raises) - Build an Automation Script

Verify that dividing by zero raises a ZeroDivisionError

Preconditions (2)

Step 1: Write a function that divides two numbers

Step 2: Call the function with the second number as zero

Step 3: Check that calling the function raises a ZeroDivisionError

✅ Expected Result: The test should pass only if ZeroDivisionError is raised when dividing by zero

Automation Requirements - pytest

Assertions Needed:

Assert that ZeroDivisionError is raised when dividing by zero

Best Practices:

Use pytest.raises context manager to catch exceptions

Keep test functions simple and focused

Name test functions starting with 'test_'

Avoid catching exceptions manually with try-except in tests

Automated Solution

PyTest

import pytest

def divide(a, b):
    return a / b

def test_divide_by_zero_raises():
    with pytest.raises(ZeroDivisionError):
        divide(10, 0)

The divide function simply divides two numbers.

The test function test_divide_by_zero_raises uses pytest.raises as a context manager to check that dividing by zero raises a ZeroDivisionError.

If the exception is raised, the test passes. If not, pytest fails the test.

This approach is clean and clear, following pytest best practices.

Common Mistakes - 3 Pitfalls

Using try-except block manually to catch exceptions in test

Not specifying the exception type in pytest.raises

{'mistake': "Writing test function names that do not start with 'test_'", 'why_bad': 'pytest will not discover or run these tests automatically.', 'correct_approach': "Name test functions starting with 'test_'."}

Bonus Challenge

Now add data-driven testing to check that dividing by zero raises ZeroDivisionError for multiple numerators: 10, -5, and 0

Show Hint