Load all pattern from all the files in folders

def _load_patterns(self, folders, pattern_dict=None):
        """Load all pattern from all the files in folders"""
        if pattern_dict is None:
            pattern_dict = {}
        for folder in folders:
            for file in os.listdir(folder):
                if regex.match('^[\w-]+$', file):
                    self._load_pattern_file(os.path.join(folder, file), pattern_dict)
        return pattern_dict