dchevell/requests-futures

Asynchronous Python HTTP Requests for Humans using Futures

Asynchronous Python HTTP Requests for Humans

Small add-on for the python requests http library. Makes use of python 3.2's concurrent.futures or the backport for prior versions of python.

The additional API and changes are minimal and strives to avoid surprises.

The following synchronous code:

from requests import Session

session = Session()
# first requests starts and blocks until finished
response_one = session.get('http://httpbin.org/get')
# second request starts once first is finished
response_two = session.get('http://httpbin.org/get?foo=bar')
# both requests are complete
print('response one status: {0}'.format(response_one.status_code))
print(response_one.content)
print('response two status: {0}'.format(response_two.status_code))
print(response_two.content)

Can be translated to make use of futures, and thus be asynchronous by creating a FuturesSession and catching the returned Future in place of Response. The Response can be retrieved by calling the result method on the Future:

from requests_futures.sessions import FuturesSession

session = FuturesSession()
# first request is started in background
future_one = session.get('http://httpbin.org/get')
# second requests is started immediately
future_two = session.get('http://httpbin.org/get?foo=bar')
# wait for the first request to complete, if it hasn't already
response_one = future_one.result()
print('response one status: {0}'.format(response_one.status_code))
print(response_one.content)
# wait for the second request to complete, if it hasn't already
response_two = future_two.result()
print('response two status: {0}'.format(response_two.status_code))
print(response_two.content)

By default a ThreadPoolExecutor is created with 2 workers. If you would like to adjust that value or share a executor across multiple sessions you can provide one to the FuturesSession constructor.

from concurrent.futures import ThreadPoolExecutor
from requests_futures.sessions import FuturesSession

session = FuturesSession(executor=ThreadPoolExecutor(max_workers=10))
# ...

As a shortcut in case of just increasing workers number you can pass max_workers straight to the FuturesSession constructor:

from requests_futures.sessions import FuturesSession
session = FuturesSession(max_workers=10)

FutureSession will use an existing session object if supplied:

from requests import session
from requests_futures.sessions import FuturesSession
my_session = session()
future_session = FuturesSession(session=my_session)

That's it. The api of requests.Session is preserved without any modifications beyond returning a Future rather than Response. As with all futures exceptions are shifted (thrown) to the future.result() call so try/except blocks should be moved there.

Canceling queued requests (a.k.a cleaning up after yourself)

If you know that you won't be needing any additional responses from futures that haven't yet resolved, it's a good idea to cancel those requests. You can do this by using the session as a context manager:

from requests_futures.sessions import FuturesSession
with FuturesSession(max_workers=1) as session:
    future = session.get('https://httpbin.org/get')
    future2 = session.get('https://httpbin.org/delay/10')
    future3 = session.get('https://httpbin.org/delay/10')
    response = future.result()

In this example, the second or third request will be skipped, saving time and resources that would otherwise be wasted.

Working in the Background

There is one additional parameter to the various request functions, background_callback, which allows you to work with the Response objects in the background thread. This can be useful for shifting work out of the foreground, for a simple example take json parsing.

from pprint import pprint
from requests_futures.sessions import FuturesSession

session = FuturesSession()

def bg_cb(sess, resp):
    # parse the json storing the result on the response object
    resp.data = resp.json()

future = session.get('http://httpbin.org/get', background_callback=bg_cb)
# do some other stuff, send some more requests while this one works
response = future.result()
print('response status {0}'.format(response.status_code))
# data will have been attached to the response object in the background
pprint(response.data)

Using ProcessPoolExecutor

Similarly to ThreadPoolExecutor, it is possible to use an instance of ProcessPoolExecutor. As the name suggest, the requests will be executed concurrently in separate processes rather than threads.

from concurrent.futures import ProcessPoolExecutor
from requests_futures.sessions import FuturesSession

session = FuturesSession(executor=ProcessPoolExecutor(max_workers=10))
# ... use as before

Hint

Using the ProcessPoolExecutor is useful, in cases where memory usage per request is very high (large response) and cycling the interpretor is required to release memory back to OS.

A base requirement of using ProcessPoolExecutor is that the Session.request, FutureSession and (the optional) background_callback all be pickle-able.

This means that only Python 3.5 is fully supported, while Python versions 3.4 and above REQUIRE an existing requests.Session instance to be passed when initializing FutureSession. Python 2.X and < 3.4 are currently not supported.

# Using python 3.4
from concurrent.futures import ProcessPoolExecutor
from requests import Session
from requests_futures.sessions import FuturesSession

session = FuturesSession(executor=ProcessPoolExecutor(max_workers=10),
                         session=Session())
# ... use as before

In case pickling fails, an exception is raised pointing to this documentation.

# Using python 2.7
from concurrent.futures import ProcessPoolExecutor
from requests import Session
from requests_futures.sessions import FuturesSession

session = FuturesSession(executor=ProcessPoolExecutor(max_workers=10),
                         session=Session())
Traceback (most recent call last):
...
RuntimeError: Cannot pickle function. Refer to documentation: https://github.com/ross/requests-futures/#using-processpoolexecutor

Important

• Python >= 3.4 required
• A session instance is required when using Python < 3.5
• If sub-classing FuturesSession it must be importable (module global)
• If using background_callback it too must be importable (module global)

Installation

pip install requests-futures