def parse_patterns(patterns_spec, ignore_patterns_spec, separator=';'):
    """
    Parses pattern argument specs and returns a two-tuple of
    (patterns, ignore_patterns).
    """
    patterns = patterns_spec.split(separator)
    ignore_patterns = ignore_patterns_spec.split(separator)
    if ignore_patterns == ['']:
        ignore_patterns = []
    return (patterns, ignore_patterns)