capsys for capturing output 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 helps you check what your code prints.

beginner

How do you access captured output using capsys?

Use capsys.readouterr() which returns a namedtuple with out (standard output) and err (standard error) strings.

beginner

Why is capturing output useful in testing?

It lets you verify that your program prints the correct messages, like logs or errors, without showing them on the screen during tests.

beginner

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

def test_greet(capsys):
    print('Hello')
    captured = capsys.readouterr()
    assert captured.out == 'Hello\n'

intermediate

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

Each call returns the output captured since the last call and clears the buffer. So, output is reset after reading.

What does capsys.readouterr() return?

A A namedtuple with <code>out</code> and <code>err</code> strings

B Only the standard output string

C Only the standard error string

D A boolean indicating if output was captured

capsys.readouterr() returns a namedtuple with two strings: out for standard output and err for standard error.

Which pytest fixture is used to capture printed output?

A capout

B stdout

C capsys

D capture

capsys is the built-in pytest fixture for capturing output.

If your code prints 'Hi' and you want to test it, what assertion checks the output correctly?

A assert captured.out == 'Hi'

B assert captured.out == 'Hi\n'

C assert captured.err == 'Hi\n'

D assert captured.out == 'hi\n'

Print adds a newline, so output ends with '\n'. The check must include it exactly.

What happens to the captured output after calling capsys.readouterr()?

A It remains unchanged

B It is saved to a file

C It doubles

D It is cleared/reset

Calling readouterr() clears the captured output buffer.

Which output streams does capsys capture?

A Both standard output and standard error

B Only standard error

C Only standard output

D Neither

capsys captures both stdout and stderr.

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

Think about how you capture and check what print shows.

Why is it important to include the newline character when asserting printed output captured by capsys?

Remember how print behaves in Python.