Python Testing with pytest

Using capsys

The capsys builtin fixture provides two bits of functionality: it allows you to retrieve stdout and stderr from some code, and it disables output capture temporarily. Let’s take a look at retrieving stdout and stderr.

Suppose you have a function to print a greeting to stdout:

ch4/cap/test_capsys.py

def greeting(name):
    print('Hi, {}'.format(name))

You can’t test it by checking the return value. You have to test stdout somehow. You can test the output by using capsys:

ch4/cap/test_capsys.py

def test_greeting(capsys):
    greeting('Earthling')
    out, err = capsys.readouterr()
    assert out == 'Hi, Earthling\n'
    assert err == ''

    greeting('Brian')
    greeting('Nerd')
    out, err = capsys.readouterr()