Conditional parametrize in PyTest - Test Execution Trace

Test Overview

This test uses pytest's parametrize decorator with a condition to run different inputs. It verifies the function returns expected results only for allowed inputs.

Test Code - pytest

PyTest

import pytest

def is_even(n):
    return n % 2 == 0

# Parametrize only even numbers
@pytest.mark.parametrize('num', [n for n in [2, 3, 4, 5] if n % 2 == 0])
def test_is_even(num):
    assert is_even(num) is True

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	Test starts	pytest test runner initialized	-	PASS
2	pytest collects test_is_even with parametrize values filtered to even numbers [2, 4]	Test cases ready for num=2 and num=4	-	PASS
3	Test runs test_is_even with num=2	Function is_even called with 2	assert is_even(2) is True	PASS
4	Test runs test_is_even with num=4	Function is_even called with 4	assert is_even(4) is True	PASS
5	Test ends with all parameterized cases passed	Test report shows 2 passed tests	-	PASS

Failure Scenario

Failing Condition: If the test runs with an odd number like 3 or 5, assertion fails

Execution Trace Quiz - 3 Questions

What values does the test actually run with?

A [2, 4]

B [2, 3, 4, 5]

C [3, 5]

D []

Key Result

Use conditional filtering in parametrize to run tests only on relevant inputs, avoiding unnecessary failures.