capsys for stdout/stderr in PyTest - Test Execution Trace

Test Overview

This test captures output printed to the console (stdout) and error messages (stderr) using pytest's capsys fixture. It verifies that the printed messages match expected text.

Test Code - pytest

PyTest

import sys

def greet():
    print("Hello, world!")
    print("Error happened", file=sys.stderr)

def test_greet_output(capsys):
    greet()
    captured = capsys.readouterr()
    assert captured.out == "Hello, world!\n"
    assert captured.err == "Error happened\n"

Execution Trace - 6 Steps

Step	Action	System State	Assertion	Result
1	Test starts	pytest test runner initialized	-	PASS
2	Calls greet() function	greet() prints 'Hello, world!' to stdout and 'Error happened' to stderr	-	PASS
3	capsys reads captured stdout and stderr	captured.out contains 'Hello, world!\n', captured.err contains 'Error happened\n'	-	PASS
4	Assert captured.out equals 'Hello, world!\n'	captured.out is 'Hello, world!\n'	assert captured.out == 'Hello, world!\n'	PASS
5	Assert captured.err equals 'Error happened\n'	captured.err is 'Error happened\n'	assert captured.err == 'Error happened\n'	PASS
6	Test ends successfully	All assertions passed	-	PASS

Failure Scenario

Failing Condition: The printed output does not match the expected strings

Execution Trace Quiz - 3 Questions

What does capsys.readouterr() return?

A An object with captured stdout and stderr strings

B A boolean indicating if output was captured

C The original print function

D A file handle to stdout

Key Result

Use pytest's capsys fixture to capture and verify console output and error messages, ensuring your code prints exactly what you expect.