Issue
I found an async httpx example where ensure_future
works but create_task
doesn't, but I can't figure out why. As I've understood that create_task
is the preferred approach, I'm wondering what's happening and how I may solve the problem.
I've been using an async httpx example at https://www.twilio.com/blog/asynchronous-http-requests-in-python-with-httpx-and-asyncio:
import asyncio
import httpx
import time
start_time = time.time()
async def get_pokemon(client, url):
resp = await client.get(url)
pokemon = resp.json()
return pokemon['name']
async def main():
async with httpx.AsyncClient() as client:
tasks = []
for number in range(1, 151):
url = f'https://pokeapi.co/api/v2/pokemon/{number}'
tasks.append(asyncio.ensure_future(get_pokemon(client, url)))
original_pokemon = await asyncio.gather(*tasks)
for pokemon in original_pokemon:
print(pokemon)
asyncio.run(main())
print("--- %s seconds ---" % (time.time() - start_time))
When run verbatim, the code produces the intended result (a list of Pokemon in less than a second). However, replacing the asyncio.ensure_future
with asyncio.create_task
instead leads to a long wait (which seems to be related to a DNS lookup timing out) and then exceptions, the first one being:
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/anyio/_core/_sockets.py", line 186, in connect_tcp
addr_obj = ip_address(remote_host)
File "/usr/lib/python3.10/ipaddress.py", line 54, in ip_address
raise ValueError(f'{address!r} does not appear to be an IPv4 or IPv6 address')
ValueError: 'pokeapi.co' does not appear to be an IPv4 or IPv6 address
Reducing the range maximum (to 70 on my computer) makes the problem disappear.
I understand https://stackoverflow.com/a/36415477/ as saying that ensure_future
and create_task
act similarly when given coroutines unless there's a custom event loop, and that create_task
is recommended.
If so, why does one of the approaches work while the other fails?
I'm using Python 3.10.5 and httpx 0.23.0.
Solution
After more debugging, I've found out that the problem lies elsewhere.
It appears that httpx doesn't use DNS precaching, so when asked to connect to the same host a bunch of times at once, it'll do a large number of DNS lookups. In turn, that caused the DNS server to fail to respond to requests some of the time.
As luck would have it, even though I tested many times, the request storm happened to make the DNS fail exactly when I was using create_task
but not when I was using ensure_future
.
In short, due to Murphy's law I was mistaking a nondeterministic problem for a deterministic one. However, it seems that httpx can in general be a bit fickle when it comes to DNS requests, for instance as reported at https://github.com/encode/httpx/discussions/2321.
Answered By - Lesser Cormorant
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