Helper method to get the national-number part of a number, formatted without any national
    prefix, and return it as a set of digit blocks that should be formatted together according to
    the formatting pattern passed in.

def _get_national_number_groups(numobj, formatting_pattern):
    """Helper method to get the national-number part of a number, formatted without any national
    prefix, and return it as a set of digit blocks that should be formatted together according to
    the formatting pattern passed in."""
    # If a format is provided, we format the NSN only, and split that according to the separator.
    nsn = national_significant_number(numobj)
    return _format_nsn_using_pattern(nsn, formatting_pattern,
                                     PhoneNumberFormat.RFC3966).split(U_DASH)