Sophie Au

Software Developer, Web Designer, Consultant, Tea Enthusiast

Mocking stdin, stderr and stdout for python unittest

05 November 2018

Working on my latest project, todoster, forced me to learn all about input/output mocking using python's built-in unittest library.

Mocking Output to stdout or stderr

The way I've been using the mock output or error stream is by getting its contents as a string. For this, there is a very simple method:


With this string you can do assertions as normal. However, since mock_err and mock_output aren't actually strings but streams you need to be very careful when trying to use the same stream a second time. The streams don't get cleared automatically so you need to either re-declare the mocks or make sure they're manually cleared out before every re-use.

If you want to have one mock for one test case (class) just make sure you're resetting mock_out and mock_err after every test like so:

import patch from unittest.mock

class TestClass
    mock_out = patch('sys.stdout', new_callable=StringIO)
    mock_err = patch('sys.stderr', new_callable=io.StringIO)

    def tearDown(self):

    # ...

On the other hand, you can mock on a test by test basis by using decorators like so:

from unittest.mock import patch

class TestClass(unittest.TestCase):

    @patch('sys.stderr', new_callable=io.StringIO)
    @patch('sys.stdout', new_callable=io.StringIO)
    test_small_functionality(self, mock_out, mock_err)
        # test something

Now you don't have to worry about clearing out the mocks after every test but it does come with having to add a new decorator for every test method that needs the mock.

Mocking stdin

If you want to mock stdin, this is the simplest solution I could find. It assumes that the code under test calls input only once though.

@patch('builtins.input', side_effect=['the input you want to test'])
def test_delete_project_say_no(self, _):
    # test something