Pytest capture STDOUT and STDERR with capsys

The capsys fixture captures everything that is printed to STDOUT and STDERR so we can compare that to the expected output and error.

• We need to include capsys as the parameter of the test function.
• The first call to readouterr will return whatever was printed to STDOUT and STDERR respectively from the start of the test function.
• The subsequent calls to readouterr will return the output that was generated since the previous call to readouterr.

from greet import welcome

def test_myoutput(capsys):
    welcome("hello", "world")
    out, err = capsys.readouterr()
    assert out == "STDOUT: hello\n"
    assert err == "STDERR: world\n"

    welcome("next")
    out, err = capsys.readouterr()
    assert out == "STDOUT: next\n"
    assert err == ""

pytest test_greet.py

Note, however, using -s won’t print anything extra in this case as the ourput was captured by capsys.

pytest -s test_greet.py