More advanced Transport API example. #984

class ThrottleTransport:
    """
    A custom httpx transport that adds some basic rate-throttling functionality.

    transport = ThrottleTransport('100/minute', **connection_pool_options)
    client = httpx.Client(transport=transport)
    """

    def __init__(self, throttle, **kwargs):
        self.pool = httpcore.SyncConnectionPool(**kwargs)
        self.history = []
        
        # Parse the thottle, which should be a string, like '100/minute'.
        count, _, duration = throttle.partition('/')
        self.max_in_history = int(count)
        self.cutoff = {'second': 1.0, 'minute': 60.0, 'hour': 3600.0}[duration]

    def request(method, url, headers, stream, timeout):
        now = time.time()

        while len(self.history) >= self.max_in_history:
            expiry = now - self.cutoff

            # Expiry old entries in the history.
            self.history = [timestamp for timestamp in self.history if timestamp > expiry]
            
            # Sleep for a bit if we've exceeded the throttle rate.
            if len(self.history) >= self.max_in_history:
                time.sleep(0.1)
                now = time.time()

        self.history.append(now)
        return self.pool.request(method, url, headers, stream, timeout)
    
    def close(self):
        self.pool.close()