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To: python-list@python.org
From: Terry Jan Reedy <tjreedy@udel.edu>
Subject: Re: newbie question about confusing exception handling in urllib
Date: Tue, 09 Apr 2013 10:19:08 -0400
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On 4/9/2013 7:41 AM, cabbar@gmail.com wrote:
> Hi,
>
> I have been using Java/Perl professionally for many years and have been trying to learn python3 recently. As my first program, I tried writing a class for a small project, and I am having really hard time understanding exception handling in urllib and in python in general...
> Basically, what I want to do is very simple,

Very funny ;-). What you are trying to do, as your first project, is 
interact with the large, multi-layered, non=deterministic monster known 
as Internet, with timeout handling, through multiple layers of library 
code. When it comes to exception handling, this is about the most 
complex thing you can do.

> try to fetch something "tryurllib.request.urlopen(request)", and:
>    - If request times out or connection is reset, re-try n times
>    - If it fails, return an error
>    - If it works return the content.
>
> But, this simple requirement became a nightmare for me. I am really confused about how I should be checking this because:
>    - When connection times out, I sometimes get URLException with "reason" field set to socket.timeout, and checking (isinstance(exception.reason, socket.timeout)) works fine
>    - But sometimes I get socket.timeout exception directly, and it has no "reason" field, so above statement fails, since there is no reason field there.

If you are curious why the different exceptions for seemingly the same 
problem, you can look at the printed traceback to see where the 
different exceptions come from. Either don't catch the exceptions, 
re-raise them, or explicitly grab the traceback (from exc_info, I believe)

>    - Connection reset is a totally different exception
>    - Not to mention, some exceptions have msg / reason / errno fields but some don't, so there is no way of knowing exception details unless you check them one by one. The only common thing I could was to find call __str__()?

The system is probably a bit more ragged then it might be if completely 
re-designed from scratch.

>    - Since, there are too many possible exceptions, you need to catch BaseException (I received URLError, socket.timeout, ConnectionRefusedError, ConnectionResetError, BadStatusLine, and none share a common parent). And, catching the top level exception is not a good thing.

You are right, catching BaseException is bad. In particular, it will 
catch KeyboardInterrupt from a user trying to stop the process. It is 
also unnecessary as all the exceptions you want to catch are derived 
from Exception, which itself is derived from BaseException.

> So, I ended up writing the following, but from everything I know, this looks really ugly and wrong???
>
>          try:
>              response = urllib.request.urlopen(request)
>              content = response.read()
>          except BaseException as ue:

except Exception as ue:

>              if (isinstance(ue, socket.timeout) or (hasattr(ue, "reason") and isinstance(ue.reason, socket.timeout)) or isinstance(ue, ConnectionResetError)):
>                  print("REQUEST TIMED OUT")
>
> or, something like:
>
>          except:

except Exception:

>              (a1,a2,a3) = sys.exc_info()
>              errorString = a2.__str__()
>              if ((errorString.find("Connection reset by peer") >= 0) or (errorString.find("error timed out") >= 0)):

--
Terry Jan Reedy