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
