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Date: Thu, 4 Jun 2015 08:00:13 +1000
Subject: Re: Retrying to send message
From: Chris Angelico <rosuav@gmail.com>
Cc: "python-list@python.org" <python-list@python.org>
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On Thu, Jun 4, 2015 at 2:15 AM, Cecil Westerhof <Cecil@decebal.nl> wrote:
>> And I'd also skip the bare except clause. If you get any sort of
>> exception, whether it's a bug, a failure from libturpial, a network
>> error, or anything else, your code will just terminate with a bland
>> and useless message. Much better to simply let the exception bubble
>> up.
>
> I kept the except. I like to see the message that went wrong. ;-)

In that case, there's an easier way to deal with it: just raise a
different exception, and let them chain. Something like this:

>>> def send_message(msg):
...     try: 1/0
...     except: raise FailedMessageException(msg)
...
>>> class FailedMessageException(Exception): pass
...
>>> send_message("Test")
Traceback (most recent call last):
  File "<stdin>", line 2, in send_message
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in send_message
__main__.FailedMessageException: Test

But I'd still avoid the bare except, and use "except Exception:"
instead. I don't think you want to catch SystemExit in this way, for
instance. So here's how I'd write that code:

>>> def send_message(msg):
...     try: 1/0
...     except Exception as e: raise FailedMessageException(msg) from e
...
>>> send_message("Test")
Traceback (most recent call last):
  File "<stdin>", line 2, in send_message
ZeroDivisionError: division by zero

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in send_message
__main__.FailedMessageException: Test

(Also, the use of "from" here causes a slightly different wording,
which I think is more appropriate. But that's minor.)

You get the report of which message failed, plus you get the complete
traceback from the original exception. Much MUCH more useful.

ChrisA