python 3.3 urllib.request

[Terry Reedy]

On 12/7/2012 12:27 PM, Hans Mulder wrote:
> On 7/12/12 13:52:52, Steeve C wrote:
>> hello,
>>
>> I have a python3 script with urllib.request which have a strange
>> behavior, here is the script :
>>
>> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>> #!/usr/bin/env python3
>> # -*- coding: utf-8 -*-
>>
>> import urllib.request
>> import sys, time
>>
>>
>> url = 'http://google.com'
>>
>> def make_some_stuff(page, url):
>>      sys.stderr.write(time.strftime("%d/%m/%Y %H:%M:%S -> page from \"")
>> + url + "\"\n")
>>      sys.stderr.write(str(page) + "\"\n")
>>      return True
>>
>> def get_page(url):
>>      while 1:
>>          try:
>>              page = urllib.request.urlopen(url)
>>              yield page
>>
>>          except urllib.error.URLError as e:
>>              sys.stderr.write(time.strftime("%d/%m/%Y %H:%M:%S ->
>> impossible to access to \"") + url + "\"\n")
>>              time.sleep(5)
>>              continue
>>
>> def main():
>>      print('in main')
>>      for page in get_page(url):
>>          make_some_stuff(page, url)
>>          time.sleep(5)
>>
>> if __name__ == '__main__':
>>      main()
>> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>
>> if the computer is connected on internet (with an ethernet connection
>> for example) and I run this script, it works like a charme :
>> - urllib.request.urlopen return the page
>> - make_some_stuff write in stderr
>> - when the ethernet cable is unplug the except block handle the error
>> while the cable is unplug, and when the cable is pluged
>> back urllib.request.urlopen return the page and make_some_stuff write in
>> stderr
>>
>> this is the normal behavior (for me, imho).
>>
>> but if the computer is not connected on internet (ethernet cable
>> unpluged) and I run this script, the except block handle the error
>> (normal), but when I plug the cable, the script continue looping
>> and urllib.request.urlopen never return the page (so, it always
>> go to the except block)
>>
>> What can I do to handle that ?

Don't do that '-).

> On my laptop, your script works as you'd hope: if I plug in the
> network cable, then the next urllib request sometimes fails, but
> the request after that succeeds.
> This is using Python 3.3 on MacOS X 10.5.
> What version are you running?
>
> What happens if you start the script with the network cable
> plugged in, then unplug it when the first request has succeeded,
> and then plug it in again when the next request has failed?

I believe he said that that worked. But unplugging cables is not a good 
idea ;-)

I remember when it was recommended that all cables be plugged in and the 
the connected devices turned on when the computer was turned on and when 
devices might not be recognized unless plugged in and on when the 
computer was booted or rebooted. In other words, ports were scanned once 
as part of the boot process and adding a device required a reboot. It 
certainly was not that long ago when I had to reboot after the Internet 
Service went down and the cable modem had to reset.

Ethernet and usb ports and modern OSes are more forgiving. But it does 
not surprise me if on some systems something has to be presence at 
process startup to evet be visible to the process.

I believe this is all beyond Python's control. So the only thing to do 
might be to change hardware and/or OS or have the program restart itself 
if it gets repeated errors.

-- 
Terry Jan Reedy