Asserting exceptions (pytest.raises) - Test Execution Trace

Test Overview

This test checks that a function raises a ValueError when given invalid input. It verifies that the exception is correctly thrown and caught using pytest.raises.

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():
    with pytest.raises(ValueError) 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 is ready to execute test_divide_by_zero	-	PASS
2	Calls divide(10, 0)	Function divide is executing with a=10, b=0	-	PASS
3	Function detects b=0 and raises ValueError("Cannot divide by zero")	Exception ValueError is raised inside divide function	-	PASS
4	pytest.raises context catches the ValueError exception	Exception is caught and stored in exc_info	Check that exception type is ValueError	PASS
5	Assert that exception message equals "Cannot divide by zero"	Exception message is accessible via exc_info.value	assert str(exc_info.value) == "Cannot divide by zero"	PASS
6	Test ends successfully	Test framework records test_divide_by_zero as passed	-	PASS

Failure Scenario

Failing Condition: The divide function does not raise ValueError when dividing by zero

Execution Trace Quiz - 3 Questions

What does pytest.raises do in this test?

A It ignores any exceptions during the test

B It prevents the exception from being raised

C It catches the expected exception to verify it was raised

D It retries the function until no exception occurs

Key Result

Use pytest.raises to confirm your code raises the right exception with the expected message. This helps catch errors early and documents expected failure cases clearly.