Limiting/throttling the rate of HTTP requests in GRequests

Question

I\'m writing a small script in Python 2.7.3 with GRequests and lxml that will allow me to gather some collectible card prices from various websites and compare them. Problem

Answer 1

I had a similar problem. Here's my solution. In your case, I would do:

def worker():
    with rate_limit('slow.domain.com', 2):
        response = requests.get('https://slow.domain.com/path')
        text = response.text
    # Use `text`

Assuming you have multiple domains you're culling from, I would setup a dictionary mapping (domain, delay) so you don't hit your rate limits.

This code assumes you're going to use gevent and monkey patch.

from contextlib import contextmanager
from gevent.event import Event
from gevent.queue import Queue
from time import time


def rate_limit(resource, delay, _queues={}):
    """Delay use of `resource` until after `delay` seconds have passed.

    Example usage:

    def worker():
        with rate_limit('foo.bar.com', 1):
            response = requests.get('https://foo.bar.com/path')
            text = response.text
        # use `text`

    This will serialize and delay requests from multiple workers for resource
    'foo.bar.com' by 1 second.

    """

    if resource not in _queues:
        queue = Queue()
        gevent.spawn(_watch, queue)
        _queues[resource] = queue

    return _resource_manager(_queues[resource], delay)


def _watch(queue):
    "Watch `queue` and wake event listeners after delay."

    last = 0

    while True:
        event, delay = queue.get()

        now = time()

        if (now - last) < delay:
            gevent.sleep(delay - (now - last))

        event.set()   # Wake worker but keep control.
        event.clear()
        event.wait()  # Yield control until woken.

        last = time()


@contextmanager
def _resource_manager(queue, delay):
    "`with` statement support for `rate_limit`."

    event = Event()
    queue.put((event, delay))

    event.wait() # Wait for queue watcher to wake us.

    yield

    event.set()  # Wake queue watcher.