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Re: Confused compare function :)

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On Thu, Dec 6, 2012 at 7:49 PM, Bruno Dupuis
<python.ml.bruno.dupuis@lisael.org> wrote:
>
> The point is Exceptions are made for error handling, not for normal
> workflow. I hate when i read that for example:
>
>     try:
>         do_stuff(mydict[k])
>     except KeyError:
>         pass
>
> (loads of them in many libraries and frameworks)
> instead of:
>
>     if k in mydict:
>         do_stuff(mydict[k])
>
> Note that the performances are better with the latter.
>

This is the age-old question of EAFP vs LBYL.

The check-first "Look Before You Leap" option has a small chance of
race condition. If something changes between the 'if' and the usage,
maybe from another thread or maybe a signal handler or perhaps some
object's __del__ method gets called or who knows what, you'll have a
problem.

Python's usual philosophy is that it's Easier to Ask Forgiveness than
Permission. Just do it, and jump out if you can't. This technique
plays *very* nicely with generic handlers. For instance, at work I
wrote an HTTP daemon that's supposed to receive XML-encoded POST data
with a particular structure. The handler function (called once for
every HTTP request) looks something like this, in Pythonesque
pseudocode:

def handler(req):
  try:
    msg = parse_xml(req.body) # returns a dict of dicts/lists/strings
    stuff = msg["soapenv:Envelope"]["soapenv:Body"]["GetItemTransactionsResponse"]
    sig = stuff["Item"]["ApplicationData"]
    if sig.does.not.match(): return "Bad request"
    sscanf(sig,"FOO %d %d %d",account,table,row)
    accountdata[account][table][row] = 1
    return "Done and successful."
  except:
    log_error_to_stderr()
    return "Done."

I don't particularly care _what_ the error is. Most of the time, I
won't even bother to look at the log file (it's run via an Upstart
job, and stderr is redirected to a file), but if I'm having problems,
I can go check. Generally, exceptions thrown by that code are the
result of malformed info packets; since it's basic HTTP, it's easy for
anyone to send a request in, and I don't care to see those errors
logged. In fact, the logging to stderr can even get commented out in
production, and used only when there's actually a problem being
diagnosed.

To try to handle all possible errors in that code by LBLY, I would
need to pepper the code with conditions and an appropriate 'return'
statement (and, though the pseudo-code has the function returning a
string, the actual code involves an explicit "send this response"
call). Plus, I'd need to predict every possible failure. With EAFP,
all I need is one simple "catch" handler for the whole block of code,
and I can easily eyeball just a few function exit points to see that
an appropriate HTTP response will always be sent.

Each has its place.

ChrisA

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Thread

Re: Confused compare function :) Bruno Dupuis <python.ml.bruno.dupuis@lisael.org> - 2012-12-06 01:19 +0100
  Re: Confused compare function :) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-12-06 00:42 +0000
    Re: Confused compare function :) Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-12-06 13:41 -0500
    Re: Confused compare function :) Anatoli Hristov <tolidtm@gmail.com> - 2012-12-06 19:55 +0100
  Re: Confused compare function :) Rotwang <sg552@hotmail.co.uk> - 2012-12-06 03:22 +0000
    Re: Confused compare function :) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-12-06 04:32 +0000
      Re: Confused compare function :) Bruno Dupuis <python.ml.bruno.dupuis@lisael.org> - 2012-12-06 09:49 +0100
        Re: Confused compare function :) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-12-06 11:47 +0000
          Re: Confused compare function :) peter <pjmakey2@gmail.com> - 2012-12-06 08:55 -0300
            Re: Confused compare function :) Hans Mulder <hansmu@xs4all.nl> - 2012-12-06 14:32 +0100
              Re: Confused compare function :) Chris Angelico <rosuav@gmail.com> - 2012-12-07 00:47 +1100
          Re: Confused compare function :) Chris Angelico <rosuav@gmail.com> - 2012-12-06 23:14 +1100
            Re: Confused compare function :) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-12-07 22:16 +0000
              Re: Confused compare function :) Terry Reedy <tjreedy@udel.edu> - 2012-12-08 02:01 -0500
              Re: Confused compare function :) Chris Angelico <rosuav@gmail.com> - 2012-12-08 18:17 +1100
              Re: Confused compare function :) MRAB <python@mrabarnett.plus.com> - 2012-12-08 17:50 +0000
            Re: Confused compare function :) Ramchandra Apte <maniandram01@gmail.com> - 2012-12-08 19:07 -0800
              Re: Confused compare function :) Chris Angelico <rosuav@gmail.com> - 2012-12-09 14:22 +1100
                Re: Confused compare function :) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-12-09 07:39 +0000
            Re: Confused compare function :) Ramchandra Apte <maniandram01@gmail.com> - 2012-12-08 19:07 -0800
          Re: Confused compare function :) Neil Cerutti <neilc@norwich.edu> - 2012-12-06 13:51 +0000
            Re: Confused compare function :) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-12-07 02:55 +0000
              Re: Confused compare function :) Neil Cerutti <neilc@norwich.edu> - 2012-12-07 16:40 +0000
        Re: Confused compare function :) Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2012-12-06 14:33 +0100
          Re: Confused compare function :) Chris Angelico <rosuav@gmail.com> - 2012-12-07 00:58 +1100
            Re: Confused compare function :) Hans Mulder <hansmu@xs4all.nl> - 2012-12-06 15:21 +0100
              Re: Confused compare function :) Chris Angelico <rosuav@gmail.com> - 2012-12-07 01:28 +1100
          Re: Confused compare function :) Anatoli Hristov <tolidtm@gmail.com> - 2012-12-06 15:22 +0100
          Re: Confused compare function :) Dave Angel <d@davea.name> - 2012-12-06 09:40 -0500
          Re: Confused compare function :) peter <pjmakey2@gmail.com> - 2012-12-06 11:46 -0300
          Re: Confused compare function :) Anatoli Hristov <tolidtm@gmail.com> - 2012-12-06 17:16 +0100
          Re: Confused compare function :) Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-12-06 16:52 +0000
          Re: Confused compare function :) Anatoli Hristov <tolidtm@gmail.com> - 2012-12-06 18:08 +0100
          Re: Confused compare function :) MRAB <python@mrabarnett.plus.com> - 2012-12-06 17:10 +0000
          Re: Confused compare function :) Anatoli Hristov <tolidtm@gmail.com> - 2012-12-06 18:31 +0100
          Re: Confused compare function :) MRAB <python@mrabarnett.plus.com> - 2012-12-06 17:52 +0000
          Re: Confused compare function :) Anatoli Hristov <tolidtm@gmail.com> - 2012-12-06 19:25 +0100
          Re: Confused compare function :) Anatoli Hristov <tolidtm@gmail.com> - 2012-12-07 14:36 +0100
        Re: Confused compare function :) Rotwang <sg552@hotmail.co.uk> - 2012-12-06 19:24 +0000
      Re: Confused compare function :) Chris Angelico <rosuav@gmail.com> - 2012-12-06 20:34 +1100
      Re: Confused compare function :) Rotwang <sg552@hotmail.co.uk> - 2012-12-06 19:25 +0000

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