Here's a nice pattern I use (I tend to change it a little each time). In general, I make a module async_utils.py and just import the top-level fetching function (e.g. here fetch_things), and then my code is free to forget about the internals (other than error handling). You can do it in other ways, but I like the 'functional' style of aiostream, and often find the repeated calls to the process function take certain defaults I set using functools.partial.
You can pass in a tqdm.tqdm progress bar to pbar (initialised with known size total=len(things)) to have it update when each async response is processed.
import asyncio
import httpx
from aiostream import stream
from functools import partial
__all__ = ["fetch", "process", "async_fetch_urlset", "fetch_things"]
async def fetch(session, url, raise_for_status=False):
response = await session.get(str(url))
if raise_for_status:
response.raise_for_status()
return response
async def process_thing(data, things, pbar=None, verbose=False):
# Map the response back to the thing it came from in the things list
source_url = data.history[0].url if data.history else data.url
thing = next(t for t in things if source_url == t.get("thing_url"))
# Handle `data.content` here, where `data` is the `httpx.Response`
if verbose:
print(f"Processing {source_url=}")
build.update({"computed_value": "result goes here"})
if pbar:
pbar.update()
async def async_fetch_urlset(urls, things, pbar=None, verbose=False, timeout_s=10.0):
timeout = httpx.Timeout(timeout=timeout_s)
async with httpx.AsyncClient(timeout=timeout) as session:
ws = stream.repeat(session)
xs = stream.zip(ws, stream.iterate(urls))
ys = stream.starmap(xs, fetch, ordered=False, task_limit=20)
process = partial(process_thing, things=things, pbar=pbar, verbose=verbose)
zs = stream.map(ys, process)
return await zs
def fetch_things(urls, things, pbar=None, verbose=False):
return asyncio.run(async_fetch_urlset(urls, things, pbar, verbose))
In this example, the input is a list of dicts (with string keys and values), things: list[dict[str,str]], and the key "thing_url" is accessed to retrieve the URL. Having a dict or object is desirable instead of just the URL string for when you want to 'map' the result back to the object it came from. The process_thing function is able to modify the input list things in-place (i.e. any changes are not scoped within the function, they change it back in the scope that called it).
You'll often find errors arise during async runs that you don't get when running synchronously, so you'll need to catch them, and re-try. A common gotcha is to retry at the wrong level (e.g. around the entire loop)
In particular, you'll want to import and catch httpcore.ConnectTimeout, httpx.ConnectTimeout, httpx.RemoteProtocolError, and httpx.ReadTimeout.
Increasing the timeout_s parameter will reduce the frequency of the timeout errors by letting the AsyncClient 'wait' for longer, but doing so may in fact slow down your program (it won't "fail fast" quite as fast).
Here's an example of how to use the async_utils module given above:
from async_utils import fetch_things
import httpx
import httpcore
# UNCOMMENT THIS TO SEE ALL THE HTTPX INTERNAL LOGGING
#import logging
#log = logging.getLogger()
#log.setLevel(logging.DEBUG)
#log_format = logging.Formatter('[%(asctime)s] [%(levelname)s] - %(message)s')
#console = logging.StreamHandler()
#console.setLevel(logging.DEBUG)
#console.setFormatter(log_format)
#log.addHandler(console)
things = [
{"url": "https://python.org", "name": "Python"},
{"url": "https://www.python-httpx.org/", "name": "HTTPX"},
]
#log.debug("URLSET:" + str(list(t.get("url") for t in things)))
def make_urlset(things):
"""Make a URL generator (empty if all have been fetched)"""
urlset = (t.get("url") for t in things if "computed_value" not in t)
return urlset
retryable_errors = (
httpcore.ConnectTimeout,
httpx.ConnectTimeout, httpx.RemoteProtocolError, httpx.ReadTimeout,
)
# ASYNCHRONOUS:
max_retries = 100
for i in range(max_retries):
print(f"Retry {i}")
try:
urlset = make_urlset(things)
foo = fetch_things(urls=urlset, things=things, verbose=True)
except retryable_errors as exc:
print(f"Caught {exc!r}")
if i == max_retries - 1:
raise
except Exception:
raise
# SYNCHRONOUS:
#for t in things:
# resp = httpx.get(t["url"])
In this example I set a key "computed_value" on a dictionary once the async response has successfully been processed which then prevents that URL from being entered into the generator on the next round (when make_urlset is called again). In this way, the generator gets progressively smaller. You can also do it with lists but I find a generator of the URLs to be pulled works reliably. For an object you'd change the dictionary key assignment/access (update/in) to attribute assignment/access (settatr/hasattr).
