Returns the size of the cache in bytes.

def size(self):
        """Returns the size of the cache in bytes."""
        total_size = 0
        for dir_path, dir_names, filenames in os.walk(self.dir):
            for f in filenames:
                fp = os.path.join(dir_path, f)
                total_size += os.path.getsize(fp)
        return total_size