How to retry urllib2.request when fails?
Question:
When urllib2.request
reaches timeout, a urllib2.URLError
exception is raised.
What is the pythonic way to retry establishing a connection?
Answers:
I would use a retry decorator. There are other ones out there, but this one works pretty well. Here’s how you can use it:
@retry(urllib2.URLError, tries=4, delay=3, backoff=2)
def urlopen_with_retry():
return urllib2.urlopen("http://example.com")
This will retry the function if URLError
is raised. Check the link above for documentation on the parameters, but basically it will retry a maximum of 4 times, with an exponential backoff delay doubling each time, e.g. 3 seconds, 6 seconds, 12 seconds.
To retry on timeout you could catch the exception as @Karl Barker suggested in the comment:
assert ntries >= 1
for i in range(1, ntries+1):
try:
page = urlopen(request, timeout=timeout)
break # success
except URLError as err:
if i == ntries or not isinstance(err.reason, socket.timeout):
raise # propagate last timeout or non-timeout errors
# use page here
There are a few libraries out there that specialize in this.
One is backoff, which is designed with a particularly functional sensibility. Decorators are passed arbitrary callables returning generators which yield successive delay values. A simple exponential backoff with a maximum retry time of 32 seconds could be defined as:
@backoff.on_exception(backoff.expo,
urllib2.URLError,
max_value=32)
def url_open(url):
return urllib2.urlopen("http://example.com")
Another is retrying which has very similar functionality but an API where retry parameters are specified by way of predefined keyword args.
For Python3, you can use urllib3.Retry
:
from urllib3 import Retry, PoolManager
retries = Retry(connect=5, read=2, redirect=5, backoff_factor=0.1)
http = PoolManager(retries=retries)
response = http.request('GET', 'http://example.com/')
If the backoff_factor is 0.1, then :func:.sleep
will sleep
for [0.0s, 0.2s, 0.4s, …] between retries. It will never be longer
than :attr:Retry.BACKOFF_MAX
.
urllib3 will sleep for::
{backoff factor} * (2 ** ({number of total retries} - 1))
When urllib2.request
reaches timeout, a urllib2.URLError
exception is raised.
What is the pythonic way to retry establishing a connection?
I would use a retry decorator. There are other ones out there, but this one works pretty well. Here’s how you can use it:
@retry(urllib2.URLError, tries=4, delay=3, backoff=2)
def urlopen_with_retry():
return urllib2.urlopen("http://example.com")
This will retry the function if URLError
is raised. Check the link above for documentation on the parameters, but basically it will retry a maximum of 4 times, with an exponential backoff delay doubling each time, e.g. 3 seconds, 6 seconds, 12 seconds.
To retry on timeout you could catch the exception as @Karl Barker suggested in the comment:
assert ntries >= 1
for i in range(1, ntries+1):
try:
page = urlopen(request, timeout=timeout)
break # success
except URLError as err:
if i == ntries or not isinstance(err.reason, socket.timeout):
raise # propagate last timeout or non-timeout errors
# use page here
There are a few libraries out there that specialize in this.
One is backoff, which is designed with a particularly functional sensibility. Decorators are passed arbitrary callables returning generators which yield successive delay values. A simple exponential backoff with a maximum retry time of 32 seconds could be defined as:
@backoff.on_exception(backoff.expo,
urllib2.URLError,
max_value=32)
def url_open(url):
return urllib2.urlopen("http://example.com")
Another is retrying which has very similar functionality but an API where retry parameters are specified by way of predefined keyword args.
For Python3, you can use urllib3.Retry
:
from urllib3 import Retry, PoolManager
retries = Retry(connect=5, read=2, redirect=5, backoff_factor=0.1)
http = PoolManager(retries=retries)
response = http.request('GET', 'http://example.com/')
If the backoff_factor is 0.1, then :func:
.sleep
will sleep
for [0.0s, 0.2s, 0.4s, …] between retries. It will never be longer
than :attr:Retry.BACKOFF_MAX
.
urllib3 will sleep for::{backoff factor} * (2 ** ({number of total retries} - 1))