[Python] Throttling Async Functions in Python Asyncio

I have a list of awaitables that I want to pass to the asyncio.AbstractEventLoop but I need to throttle the requests to a third party API.

I would like to avoid something that waits to pass the future to the loop because in the meantime I block my loop waiting. What options do I have? Semaphores and ThreadPools will limit how many are running concurrently, but that's not my problem. I need to throttle my requests to 100/sec, but it doesn't matter how long it takes to complete the request.

This is a very concise (non)working example using the standard library, that demonstrates the problem. This is supposed to throttle at 100/sec but throttles at 116.651/sec. What's the best way to throttle the scheduling of an asynchronous request in asyncio?

Working code:

import asyncio
from threading import Lock

class PTBNL:

def __init__(self):
self._req_id_seq = 0
self._futures = {}
self._results = {}
self.token_bucket = TokenBucket()
self.token_bucket.set_rate(100)

def run(self, *awaitables):

loop = asyncio.get_event_loop()

if not awaitables:
loop.run_forever()
elif len(awaitables) == 1:
return loop.run_until_complete(*awaitables)
else:
future = asyncio.gather(*awaitables)
return loop.run_until_complete(future)

def sleep(self, secs) -> True:

self.run(asyncio.sleep(secs))
return True

def get_req_id(self) -> int:

new_id = self._req_id_seq
self._req_id_seq += 1
return new_id

def start_req(self, key):

loop = asyncio.get_event_loop()
future = loop.create_future()
self._futures[key] = future
return future

def end_req(self, key, result=None):

future = self._futures.pop(key, None)
if future:
if result is None:
result = self._results.pop(key, [])
if not future.done():
future.set_result(result)

def req_data(self, req_id, obj):
# Do Some Work Here
self.req_data_end(req_id)
pass

def req_data_end(self, req_id):
print(req_id, " has ended")
self.end_req(req_id)

async def req_data_async(self, obj):

req_id = self.get_req_id()
future = self.start_req(req_id)

self.req_data(req_id, obj)

await future
return future.result()

async def req_data_batch_async(self, contracts):

futures = []
FLAG = False

for contract in contracts:
req_id = self.get_req_id()
future = self.start_req(req_id)
futures.append(future)

nap = self.token_bucket.consume(1)

if FLAG is False:
FLAG = True
start = asyncio.get_event_loop().time()

asyncio.get_event_loop().call_later(nap, self.req_data, req_id, contract)

await asyncio.gather(*futures)
elapsed = asyncio.get_event_loop().time() - start

return futures, len(contracts)/elapsed

class TokenBucket:

def __init__(self):
self.tokens = 0
self.rate = 0
self.last = asyncio.get_event_loop().time()
self.lock = Lock()

def set_rate(self, rate):
with self.lock:
self.rate = rate
self.tokens = self.rate

def consume(self, tokens):
with self.lock:
if not self.rate:
return 0

now = asyncio.get_event_loop().time()
lapse = now - self.last
self.last = now
self.tokens += lapse * self.rate

if self.tokens > self.rate:
self.tokens = self.rate

self.tokens -= tokens

if self.tokens >= 0:
return 0
else:
return -self.tokens / self.rate


if __name__ == '__main__':

asyncio.get_event_loop().set_debug(True)
app = PTBNL()

objs = [obj for obj in range(500)]

l,t = app.run(app.req_data_batch_async(objs))

print(l)
print(t)


Edit: I've added a simple example of TrottleTestApp here using semaphores, but still can't throttle the execution:

import asyncio
import time


class ThrottleTestApp:

def __init__(self):
self._req_id_seq = 0
self._futures = {}
self._results = {}
self.sem = asyncio.Semaphore()

async def allow_requests(self, sem):
"""Permit 100 requests per second; call
loop.create_task(allow_requests())
at the beginning of the program to start this routine. That call returns
a task handle that can be canceled to end this routine.

asyncio.Semaphore doesn't give us a great way to get at the value other
than accessing sem._value. We do that here, but creating a wrapper that
adds a current_value method would make this cleaner"""

while True:
while sem._value < 100: sem.release()
await asyncio.sleep(1) # Or spread more evenly
# with a shorter sleep and
# increasing the value less

async def do_request(self, req_id, obj):
await self.sem.acquire()

# this is the work for the request
self.req_data(req_id, obj)

def run(self, *awaitables):

loop = asyncio.get_event_loop()

if not awaitables:
loop.run_forever()
elif len(awaitables) == 1:
return loop.run_until_complete(*awaitables)
else:
future = asyncio.gather(*awaitables)
return loop.run_until_complete(future)

def sleep(self, secs: [float]=0.02) -> True:

self.run(asyncio.sleep(secs))
return True

def get_req_id(self) -> int:

new_id = self._req_id_seq
self._req_id_seq += 1
return new_id

def start_req(self, key):

loop = asyncio.get_event_loop()
future = loop.create_future()
self._futures[key] = future
return future

def end_req(self, key, result=None):

future = self._futures.pop(key, None)
if future:
if result is None:
result = self._results.pop(key, [])
if not future.done():
future.set_result(result)

def req_data(self, req_id, obj):
# This is the method that "does" something
self.req_data_end(req_id)
pass

def req_data_end(self, req_id):

print(req_id, " has ended")
self.end_req(req_id)

async def req_data_batch_async(self, objs):

futures = []
FLAG = False

for obj in objs:
req_id = self.get_req_id()
future = self.start_req(req_id)
futures.append(future)

if FLAG is False:
FLAG = True
start = time.time()

self.do_request(req_id, obj)

await asyncio.gather(*futures)
elapsed = time.time() - start
print("Roughly %s per second" % (len(objs)/elapsed))

return futures


if __name__ == '__main__':

asyncio.get_event_loop().set_debug(True)
app = ThrottleTestApp()

objs = [obj for obj in range(10000)]

app.run(app.req_data_batch_async(objs))

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