def __init__(self, tokens, fill_rate):
        """tokens is the total tokens in the bucket. fill_rate is the
        rate in tokens/second that the bucket will be refilled."""
        self.capacity = float(tokens)
        self._tokens = float(tokens)
        self.fill_rate = float(fill_rate)
        self.timestamp = time()
tokens is the total tokens in the bucket. fill_rate is the
        rate in tokens/second that the bucket will be refilled.
recipes/Python/511490_Implementatitoken_bucket/recipe-511490.py