Matching exception messages in PyTest - Test Execution Trace

Test Overview

This test checks that a function raises a ValueError with the exact expected message when given invalid input.

Test Code - pytest

PyTest

import pytest

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

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

Execution Trace - 6 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	Function raises ValueError with message 'Cannot divide by zero'	Exception raised inside divide function	pytest checks exception type is ValueError	PASS
4	pytest matches exception message with 'Cannot divide by zero'	Exception message available for matching	Exception message matches exactly	PASS
5	Assertion checks exception message string equals 'Cannot divide by zero'	Exception info captured in exc_info	assert str(exc_info.value) == 'Cannot divide by zero'	PASS
6	Test ends successfully	All assertions passed	-	PASS

Failure Scenario

Failing Condition: The divide function raises ValueError with a different message or no exception

Execution Trace Quiz - 3 Questions

What does the 'match' parameter in pytest.raises do?

A It ignores the exception message

B It checks that the exception message contains the given string

C It changes the exception message

D It catches any exception type

Key Result

Always verify both the exception type and the exact message to ensure your code fails as expected with clear error information.