How to test a URL request in a "while True" loop

Philip Semanchuk philip at semanchuk.com
Wed Dec 30 19:31:41 CET 2009


On Dec 30, 2009, at 11:00 AM, Brian D wrote:

> I'm actually using mechanize, but that's too complicated for testing
> purposes. Instead, I've simulated in a urllib2 sample below an attempt
> to test for a valid URL request.
>
> I'm attempting to craft a loop that will trap failed attempts to
> request a URL (in cases where the connection intermittently fails),
> and repeat the URL request a few times, stopping after the Nth attempt
> is tried.
>
> Specifically, in the example below, a bad URL is requested for the
> first and second iterations. On the third iteration, a valid URL will
> be requested. The valid URL will be requested until the 5th iteration,
> when a break statement is reached to stop the loop. The 5th iteration
> also restores the values to their original state for ease of repeat
> execution.
>
> What I don't understand is how to test for a valid URL request, and
> then jump out of the "while True" loop to proceed to another line of
> code below the loop. There's probably faulty logic in this approach. I
> imagine I should wrap the URL request in a function, and perhaps store
> the response as a global variable.
>
> This is really more of a basic Python logic question than it is a
> urllib2 question.

Hi Brian,
While I don't fully understand what you're trying to accomplish by  
changing the URL to google.com after 3 iterations, I suspect that some  
of your trouble comes from using "while True". Your code would be  
clearer if the while clause actually stated the exit condition. Here's  
a suggestion (untested):

MAX_ATTEMPTS = 5

count = 0
while count <= MAX_ATTEMPTS:
    count += 1
    try:
       print 'attempt ' + str(count)
       request = urllib2.Request(url, None, headers)
       response = urllib2.urlopen(request)
       if response:
          print 'True response.'
    except URLError:
       print 'fail ' + str(count)



You could also save the results  (untested):

MAX_ATTEMPTS = 5

count = 0
results = [ ]
while count <= MAX_ATTEMPTS:
    count += 1
    try:
       print 'attempt ' + str(count)
       request = urllib2.Request(url, None, headers)
       f = urllib2.urlopen(request)
       # Note that here I ignore the doc that says "None may be
       # returned if no handler handles the request". Caveat emptor.
       results.append(f.info())
       f.close()
    except URLError:
       # Even better, append actual reasons for the failure.
       results.append(False)

for result in results:
    print result

I guess if you're going to do the same number of attempts each time, a  
for loop would be more expressive, but you probably get the idea.


Hope this helps
Philip



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