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Re: Awsome Python - chained exceptions

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Date Wed, 13 Feb 2013 17:58:46 +1100
Subject Re: Awsome Python - chained exceptions
From Chris Angelico <rosuav@gmail.com>
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On Wed, Feb 13, 2013 at 1:47 PM, Rick Johnson
<rantingrickjohnson@gmail.com> wrote:
>On Tuesday, February 12, 2013 12:15:29 AM UTC-6, Steven D'Aprano wrote:
>> If you've ever written an exception handler, you've probably written a
>> *buggy* exception handler:
>>
>> def getitem(items, index):
>>     # One-based indexing.
>>     try:
>>         return items[index-1]
>>     except IndexError:
>>         print ("Item at index %d is missing" % index - 1)  # Oops!
>>
>>
>> Unfortunately, when an exception occurs inside an except or finally
>> block, the second exception masks the first, and the reason for the
>> original exception is lost:
>>
>> py> getitem(['one', 'two', 'three'], 5)  # Python 2.6
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in <module>
>>   File "<stdin>", line 6, in getitem
>> TypeError: unsupported operand type(s) for -: 'str' and 'int'
>
> Which (by showing the offensive line) is quite clear to me.

No, the offending (not offensive) line is "return items[index-1]",
which doesn't feature in your traceback at all. It DOES, however,
feature in the Py3.1 double traceback (it's listed as line 4)..

> 1. You are using the print function (so we can assume you are using Python 3.x)

He is? Could just as easily be the print statement with a single
argument, with unnecessary parentheses around that argument. Which, if
I recall correctly, is one of the recommended approaches for making
2/3 bi-compatible code.

> but then you go and use that old ugly "%" string interpolation syntax crap! when you should have used the format method of strings.
>
>     print("Item at index {0} is missing".format(index-1)) # Oops!
>
> ...Oh Steven, if you only knew how we interpreted the "Oops!", more like "Doh!".

No. Definitely not. Percent interpolation isn't going anywhere - core
devs have said so - and there are many occasions when it is at least
as well suited to the task as .format() is. Also, it's a notation
that's well understood *across languages* and in a variety of message
interpolation systems. Anyone who's worked with *any* of them will
understand that %s inserts a string, %d a number (in decimal), etc,
etc. Granted, the exact details after that may change (eg Python has
%r to emit the representation, while Pike uses %O for "any object",
with similar notation), but the format specifiers and modifiers that
came from C are fairly stable, readable, and compact.

In what way is a trivial example like this improved by the use of
format()? The ONLY thing I can think of is that, by forcing you to put
parentheses around the argument, it avoids the issue from the original
post, which is one of operator precedence - but that's something
that's fairly easy to spot when you know what you're looking for, and
is definitely not specific to string formatting.

ChrisA

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Thread

Awsome Python - chained exceptions Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-12 06:15 +0000
  Re: Awsome Python - chained exceptions Terry Reedy <tjreedy@udel.edu> - 2013-02-12 10:13 -0500
  Re: Awsome Python - chained exceptions Zero Piraeus <schesis@gmail.com> - 2013-02-12 14:01 -0400
    Re: Awsome Python - chained exceptions Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-12 19:01 -0800
      Re: Awsome Python - chained exceptions Ian Kelly <ian.g.kelly@gmail.com> - 2013-02-14 00:00 -0700
        Re: Awsome Python - chained exceptions alex23 <wuwei23@gmail.com> - 2013-02-14 16:10 -0800
    Re: Awsome Python - chained exceptions Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-12 19:01 -0800
  Re: Awsome Python - chained exceptions Ethan Furman <ethan@stoneleaf.us> - 2013-02-12 10:15 -0800
  Re: Awsome Python - chained exceptions Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-12 18:47 -0800
    Re: Awsome Python - chained exceptions Michael Torrie <torriem@gmail.com> - 2013-02-12 21:18 -0700
    Re: Awsome Python - chained exceptions Chris Angelico <rosuav@gmail.com> - 2013-02-13 17:58 +1100
      Re: Awsome Python - chained exceptions Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-13 08:14 -0800
        Re: Awsome Python - chained exceptions Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-13 08:19 -0800
        Re: Awsome Python - chained exceptions Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-13 08:19 -0800
        Re: Awsome Python - chained exceptions Chris Angelico <rosuav@gmail.com> - 2013-02-14 09:10 +1100
          Re: Awsome Python - chained exceptions Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-14 06:39 +0000
            Re: Awsome Python - chained exceptions Serhiy Storchaka <storchaka@gmail.com> - 2013-02-15 20:51 +0200
        Re: Awsome Python - chained exceptions Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com> - 2013-02-14 13:01 +0100
          Re: Awsome Python - chained exceptions Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-14 18:56 -0800
            Re: Awsome Python - chained exceptions Chris Angelico <rosuav@gmail.com> - 2013-02-15 17:18 +1100
              Re: Awsome Python - chained exceptions Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-14 23:51 -0800
                Re: Awsome Python - chained exceptions Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com> - 2013-02-15 10:00 +0100
              Re: Awsome Python - chained exceptions Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-14 23:51 -0800
                Re: Awsome Python - chained exceptions alex23 <wuwei23@gmail.com> - 2013-02-17 17:35 -0800
                Re: Awsome Python - chained exceptions Dave Angel <davea@davea.name> - 2013-02-17 20:48 -0500
                Re: Awsome Python - chained exceptions Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-17 21:51 -0800
                news.gmane.org (was Re: Awsome Python - chained exceptions Terry Reedy <tjreedy@udel.edu> - 2013-02-18 01:10 -0500
                Re: news.gmane.org (was Re: Awsome Python - chained exceptions rurpy@yahoo.com - 2013-02-18 09:12 -0800
                Re: news.gmane.org (was Re: Awsome Python - chained exceptions Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-18 18:32 +0000
                Re: news.gmane.org (was Re: Awsome Python - chained exceptions Terry Reedy <tjreedy@udel.edu> - 2013-02-18 17:45 -0500
                Re: news.gmane.org (was Re: Awsome Python - chained exceptions Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-18 06:30 +0000
                Re: news.gmane.org (was Re: Awsome Python - chained exceptions Kwpolska <kwpolska@gmail.com> - 2013-02-18 16:27 +0100
                Re: Awsome Python - chained exceptions alex23 <wuwei23@gmail.com> - 2013-02-18 18:18 -0800
                Re: Awsome Python - chained exceptions rurpy@yahoo.com - 2013-02-19 07:52 -0800
                Re: Awsome Python - chained exceptions rusi <rustompmody@gmail.com> - 2013-02-19 09:14 -0800
                Re: Awsome Python - chained exceptions alex23 <wuwei23@gmail.com> - 2013-02-19 22:57 -0800
                Re: Awsome Python - chained exceptions Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-20 22:50 +1100
                Re: Awsome Python - chained exceptions Rotwang <sg552@hotmail.co.uk> - 2013-02-20 16:00 +0000
                RE: Awsome Python - chained exceptions "J. Marc Edwards" <marc.edwards@nimbisservices.com> - 2013-02-20 11:08 -0500
                Re: Awsome Python - chained exceptions rurpy@yahoo.com - 2013-02-20 08:13 -0800
                Re: Awsome Python - chained exceptions Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-02-20 18:49 -0500
                Re: Awsome Python - chained exceptions Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-02-20 18:47 -0500
      Re: Awsome Python - chained exceptions Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-13 08:14 -0800
    Re: Awsome Python - chained exceptions Michael Torrie <torriem@gmail.com> - 2013-02-13 00:22 -0700

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