def test_call_with_arg(mock_sub, capsys):
    'Any extra keyword arguments are passed through as-is to check_output.'
    output = mock_sub.parse_output(splitlines_parser, ['hello', 'world'], extra_arg='asdf')
    mock_sub._subprocess.check_output.assert_called_with(['hello', 'world'], extra_arg='asdf', text=True)
    assert (capsys.readouterr().out == '')
    assert (output == ['some output line 1', 'more output line 2'])
Any extra keyword arguments are passed through as-is to check_output.
def test_call_with_arg(mock_sub, capsys):
    
    output = mock_sub.parse_output(splitlines_parser, ['hello', 'world'], extra_arg='asdf')
    mock_sub._subprocess.check_output.assert_called_with(['hello', 'world'], extra_arg='asdf', text=True)
    assert (capsys.readouterr().out == )
    assert (output == ['some output line 1', 'more output line 2'])