Testing custom exceptions in PyTest - Test Execution Trace

Test Overview

This test checks if a custom exception is raised correctly when a function receives invalid input. It verifies that the exception type and message are as expected.

Test Code - pytest

PyTest

import pytest

class CustomError(Exception):
    pass

def divide(a, b):
    if b == 0:
        raise CustomError("Cannot divide by zero")
    return a / b

def test_divide_raises_custom_error():
    with pytest.raises(CustomError) as exc_info:
        divide(10, 0)
    assert str(exc_info.value) == "Cannot divide by zero"

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	Test starts	pytest test runner initialized	-	PASS
2	Calls divide(10, 0)	Function divide is executing with a=10, b=0	-	PASS
3	Raises CustomError with message 'Cannot divide by zero'	Exception raised inside divide function	pytest.raises context captures CustomError	PASS
4	Checks exception message equals 'Cannot divide by zero'	Exception message available in exc_info.value	assert str(exc_info.value) == 'Cannot divide by zero'	PASS
5	Test ends	Test passed successfully	-	PASS

Failure Scenario

Failing Condition: Function divide does not raise CustomError when dividing by zero

Execution Trace Quiz - 3 Questions

What does the test verify about the divide function?

A It raises ValueError when dividing by zero

B It returns zero when dividing by zero

C It raises CustomError when dividing by zero

D It returns None when dividing by zero

Key Result

Always use pytest.raises context manager to verify that your code raises the expected custom exceptions with the correct message.