Python: how to avoid code duplication in exception catching?

Question

What is a good pattern to avoid code duplication when dealing with different exception types in Python, eg. I want to treat URLError and HTTPError simlar but not quite:

try:
    page = urlopen(request)
except URLError, err:
    logger.error("An error ocurred %s", err)
except HTTPError, err:
    logger.error("An error occured %s", err)
    logger.error("Error message: %s", err.read())

In this example, I would like to avoid the duplication of the first logger.error call. Given URLError is the parent of HTTPError one could do something like this:

except URLError, err:
    logger.error("An error occurred %s", err)
    try:
         raise err
    except HTTPError, err:
         # specialization for http errors
         logger.error("Error message: %s", err.read())
    except:
        pass

Another approach would be to use isinstance eg. if URLError and HTTPError would not be in a chain of inheritance:

except (URLError, HTTPError), err:
    logger.error("An error occured %s", err)
    if isinstance(err, HTTPError):
         logger.error("Error message: %s", err.read())

Which one should I prefer, is there another better approach?

Tim Pietzcker · Accepted Answer · 2011-10-10 12:37:01Z

10

I think that your third example is the best solution.

It's the shortest version
It avoids duplication
It is clear to read and easy to follow, much unlike the second version.

You might want to use the newer except FooError as err syntax, though, if you're on Python 2.6 or higher.

Also, in your example, the first version isn't quite correct since the URLError handler already catches the HTTPError, so the except HTTPError part is never reached. You'd have to switch the two excepts around. Another reason not to use this.

edited Oct 10, 2011 at 12:37

answered Oct 10, 2011 at 12:25

Tim Pietzcker

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1 Comment

Bernhard Over a year ago

Thanks Tim! That's convincing. Great you spotted the URLError, HTTPError ordering in the first example. I don't edit the question as it clearly indicates the problem with that example as you mentioned in your answer.

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