def pytest_assertrepr_compare(config, op, left, right):
    """return explanation for comparisons in failing assert expressions.

    Return None for no custom explanation, otherwise return a list
    of strings.  The strings will be joined by newlines but any newlines
    *in* a string will be escaped.  Note that all but the first line will
    be indented slightly, the intention is for the first line to be a summary.

    :param _pytest.config.Config config: pytest config object
    """
