def METHOD_NAME(input, expected):
    """
    Verify that `expand_vars` expands variables in configuration.
    """
    def create_source(variables):
        return lambda name: variables[name]
    sources = {
        "SRC_1": create_source(
            {
                "PASSWORD_1": "my-password",
                "PASSWORD_2": "super-secret",
                "PÄSSWÖRD_3": "non-ascii-secret",
                "/path/to/password.txt": "file-contents",
            }
        ),
        "SRC_2": create_source(
            {
                "PASSWORD_1": "p4ssw0rd",
            }
        ),
    }
    expanded = mqttwarn.configuration.expand_vars(input, sources)
    assert expanded == expected