Receives the argument (from the constructor), and normalizes it
        into a list of Pattern objects.

def _preprocess(argument, casesensitive):
        """Receives the argument (from the constructor), and normalizes it
        into a list of Pattern objects."""
        pattern_set = PatternSet()
        if argument is not None:
            if isinstance(argument, STRING_TYPES):
                argument = [argument, ]

            for glob in argument:
                if isinstance(glob, str):
                    patterns = Pattern.create(glob, casesensitive)
                    pattern_set.extend(patterns)

                elif isinstance(glob, Pattern):
                    pattern_set.append(glob)

        return pattern_set