pytest capsys: checking output AND getting it reported?

Question

Python 3.4.1, pytest 2.6.2.

When a test fails, pytest will routinely report what was printed to stdout by the test. For instance this code:

Answer 1

Asides using "startswith".You can also use the "in" keyword, for example:

    assert "Hello" in output

This is great if you have a huge amount of data being passed to stdout, you can use "in" to check for different lines in your stdout.

def test_result_and_stdout(capsys):
    result = method_under_test()
    out, err = capsys.readouterr()
    sys.stdout.write(out)
    sys.stderr.write(err)
    assert "Hello" in out 
    assert result == 42

You can also assert what's passed into stderr instead of stdout by using:

    assert "What you are expecting" in err

Also note that line:

    out, err = capsys.readouterr()

creates a snapshot of the output to both stdout and stderr so far so you can assert what you are expecting for that test.