The following construct used pervasively in tests/test_config.py:

    with pytest.raises(subprocess.CalledProcessError) as e:
        cmd('...')
        assert 'error string' in str(e)

but this doesn't work for two reasons:

1. The assert statement must be outside the context manager (i.e. dedented one level) for it to execute at all if the exception is raised
2. The error string is very often not part of the exception object, so one needs to check the captured stderr instead

Something like this works instead:

    with pytest.raises(subprocess.CalledProcessError) as exc_info:
        cmd('...', stderr=subprocess.STDOUT)
    err_msg = exc_info.value.output.decode("utf-8")
    assert 'str' in err_msg