capsys for stdout/stderr in PyTest - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is capsys in pytest?

capsys is a pytest fixture that captures output sent to stdout and stderr during test execution. It allows you to check what your code prints or logs.

beginner

How do you access captured stdout and stderr using capsys?

Use capsys.readouterr() which returns a namedtuple with out (stdout) and err (stderr) strings.

beginner

Why is capturing stdout and stderr useful in tests?

It helps verify that your program prints the correct messages or error logs, similar to checking a receipt after a purchase to confirm details.

beginner

Show a simple pytest test function using capsys to check printed output.

def test_print_message(capsys):
    print('Hello, world!')
    captured = capsys.readouterr()
    assert captured.out == 'Hello, world!\n'

intermediate

What happens if you call capsys.readouterr() multiple times in the same test?

Each call returns the output captured since the last call, then clears the buffer. This lets you check output in parts.

What does capsys.readouterr() return?

A A namedtuple with <code>stdout</code> and <code>stderr</code> output

B Only the <code>stdout</code> output

C Only the <code>stderr</code> output

D A boolean indicating if output was captured

capsys.readouterr() returns a namedtuple with two strings: out for stdout and err for stderr.

Why use capsys in pytest tests?

A To mock database calls

B To capture and check printed output

C To speed up test execution

D To generate test data

capsys is used to capture printed output so tests can verify it.

If your code prints 'Error!' to stderr, how do you check it with capsys?

A Use <code>assert False</code>

B Check <code>captured.out</code> after <code>capsys.readouterr()</code>

C Use <code>print()</code> in the test

D Check <code>captured.err</code> after <code>capsys.readouterr()</code>

stderr output is stored in captured.err.

What happens if you forget to call capsys.readouterr() in your test?

A Output is printed to the console

B The test will fail automatically

C You cannot access the captured output

D The test runs faster

You must call capsys.readouterr() to get the captured output strings.

Can capsys capture output from functions called inside your test?

A Yes, it captures all output during the test

B No, only output from the test function itself

C Only output from imported modules

D Only output from print statements in the test

capsys captures all output sent to stdout and stderr during the test, including from called functions.

Explain how to use capsys in a pytest test to verify printed output.

Think about how you capture and check what your code prints.

Describe the difference between stdout and stderr and how capsys helps test both.

Consider normal messages vs error messages.