What is the best way to verify that a specific set of named keyword argument parameters of a method are not None, and be able to refer to those parameters, by name, in a log message reporting the missing parameters, but not causing the method to explicitly fail?

demand_not_none_keyword_args.py

def _possibly_deprecated_someday(
        required_a,
        required_b,
        named_only_for_readability_a=None,
        named_only_for_readability_b=None,
        named_only_for_readability_c=None,
        named_only_for_readability_d=None):
    """Does some complicated stuff."""

    required_keywords = [
        'named_only_for_readability_a',
        'named_only_for_readability_b',
        'named_only_for_readability_c',
        'named_only_for_readability_d'
    ]
    missing_parameters = [x for x in required_keywords if eval(x) is None]

    if len(missing_parameters) > 0:
        log.warning('Missing %s parameter(s) for '
                    'base.module#_possibly_deprecated_someday(...). '
                    'Returning -1.0.' % ", ".join(missing_parameters))
        return -1.0 # Which means "N/A"

    complex_stuff = []

    return complex_stuff

demo_solution.py

def log_missing(method, failure_case, *parameters):
    log.warning('Missing %s parameter(s) for %s.' % (", ".join(parameters), method))
    return failure_case()

@ensure_keyword_arguments_are_present(failure_case=lambda {
    return -1.0 # Which means "N/A".
})
def _possibly_deprecated_someday(
        required_a,
        required_b,
        named_only_for_readability_a=None,
        named_only_for_readability_b=None,
        named_only_for_readability_c=None,
        named_only_for_readability_d=None):
    """Does some complicated stuff."""

    complex_stuff = []

    return complex_stuff