[python] Can I set max_retries for requests.request?

The Python requests module is simple and elegant but one thing bugs me. It is possible to get a requests.exception.ConnectionError with a message like:

Max retries exceeded with url: ...

This implies that requests can attempt to access the data several times. But there is not a single mention of this possibility anywhere in the docs. Looking at the source code I didn't find any place where I could alter the default (presumably 0) value.

So is it possible to somehow set the maximum number of retries for requests?

This question is related to python python-requests

The answer is


Be careful, Martijn Pieters's answer isn't suitable for version 1.2.1+. You can't set it globally without patching the library.

You can do this instead:

import requests
from requests.adapters import HTTPAdapter

s = requests.Session()
s.mount('http://www.github.com', HTTPAdapter(max_retries=5))
s.mount('https://www.github.com', HTTPAdapter(max_retries=5))

A cleaner way to gain higher control might be to package the retry stuff into a function and make that function retriable using a decorator and whitelist the exceptions.

I have created the same here: http://www.praddy.in/retry-decorator-whitelisted-exceptions/

Reproducing the code in that link :

def retry(exceptions, delay=0, times=2):
"""
A decorator for retrying a function call with a specified delay in case of a set of exceptions

Parameter List
-------------
:param exceptions:  A tuple of all exceptions that need to be caught for retry
                                    e.g. retry(exception_list = (Timeout, Readtimeout))
:param delay: Amount of delay (seconds) needed between successive retries.
:param times: no of times the function should be retried


"""
def outer_wrapper(function):
    @functools.wraps(function)
    def inner_wrapper(*args, **kwargs):
        final_excep = None  
        for counter in xrange(times):
            if counter > 0:
                time.sleep(delay)
            final_excep = None
            try:
                value = function(*args, **kwargs)
                return value
            except (exceptions) as e:
                final_excep = e
                pass #or log it

        if final_excep is not None:
            raise final_excep
    return inner_wrapper

return outer_wrapper

@retry(exceptions=(TimeoutError, ConnectTimeoutError), delay=0, times=3)
def call_api():

This will not only change the max_retries but also enable a backoff strategy which makes requests to all http:// addresses sleep for a period of time before retrying (to a total of 5 times):

import requests
from urllib3.util.retry import Retry
from requests.adapters import HTTPAdapter

s = requests.Session()

retries = Retry(total=5,
                backoff_factor=0.1,
                status_forcelist=[ 500, 502, 503, 504 ])

s.mount('http://', HTTPAdapter(max_retries=retries))

s.get('http://httpstat.us/500')

As per documentation for Retry: if the backoff_factor is 0.1, then sleep() will sleep for [0.1s, 0.2s, 0.4s, ...] between retries. It will also force a retry if the status code returned is 500, 502, 503 or 504.

Various other options to Retry allow for more granular control:

  • total – Total number of retries to allow.
  • connect – How many connection-related errors to retry on.
  • read – How many times to retry on read errors.
  • redirect – How many redirects to perform.
  • method_whitelist – Set of uppercased HTTP method verbs that we should retry on.
  • status_forcelist – A set of HTTP status codes that we should force a retry on.
  • backoff_factor – A backoff factor to apply between attempts.
  • raise_on_redirect – Whether, if the number of redirects is exhausted, to raise a MaxRetryError, or to return a response with a response code in the 3xx range.
  • raise_on_status – Similar meaning to raise_on_redirect: whether we should raise an exception, or return a response, if status falls in status_forcelist range and retries have been exhausted.

NB: raise_on_status is relatively new, and has not made it into a release of urllib3 or requests yet. The raise_on_status keyword argument appears to have made it into the standard library at most in python version 3.6.

To make requests retry on specific HTTP status codes, use status_forcelist. For example, status_forcelist=[503] will retry on status code 503 (service unavailable).

By default, the retry only fires for these conditions:

  • Could not get a connection from the pool.
  • TimeoutError
  • HTTPException raised (from http.client in Python 3 else httplib). This seems to be low-level HTTP exceptions, like URL or protocol not formed correctly.
  • SocketError
  • ProtocolError

Notice that these are all exceptions that prevent a regular HTTP response from being received. If any regular response is generated, no retry is done. Without using the status_forcelist, even a response with status 500 will not be retried.

To make it behave in a manner which is more intuitive for working with a remote API or web server, I would use the above code snippet, which forces retries on statuses 500, 502, 503 and 504, all of which are not uncommon on the web and (possibly) recoverable given a big enough backoff period.

EDITED: Import Retry class directly from urllib3.


After struggling a bit with some of the answers here, I found a library called backoff that worked better for my situation. A basic example:

import backoff

@backoff.on_exception(
    backoff.expo,
    requests.exceptions.RequestException,
    max_tries=5,
    giveup=lambda e: e.response is not None and e.response.status_code < 500
)
def publish(self, data):
    r = requests.post(url, timeout=10, json=data)
    r.raise_for_status()

I'd still recommend giving the library's native functionality a shot, but if you run into any problems or need broader control, backoff is an option.