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Re: Misleading error message of the day

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From Robert Kern <robert.kern@gmail.com>
Subject Re: Misleading error message of the day
Date Thu, 08 Dec 2011 19:57:15 +0000
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On 12/8/11 4:21 PM, Chris Angelico wrote:
> On Fri, Dec 9, 2011 at 2:55 AM, Andrea Crotti<andrea.crotti.0@gmail.com>  wrote:
>> Yes but how do you know how many values you generated when it quits?
>> I mean I don't know how it work internally, but it should keep a temporary
>> list of the yielded values to be able to find out how many values are
>> there..
>
> Iterator unpacking works roughly thus:
>
> 1) Count up how many results you need (call that N)
> 2) N times, get a value from the iterator. If StopIteration is raised,
> swallow it and raise ValueError because there were too few values.
> 3) Attempt to get one more value from the iterator. If StopIteration
> is NOT raised, raise ValueError because there were too many values.
>
> At no point is the "total size" of the iterator counted (it could,
> after all, be infinite). When ValueError is raised, all that's known
> is that StopIteration wasn't raised at the end of the process.

unpack_iterable() has the original object available to it, not just the 
iterator. It could opportunistically check for __len__() and fall back to the 
less informative message when it is absent.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco

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Thread

Misleading error message of the day Roy Smith <roy@panix.com> - 2011-12-08 09:23 -0500
  Re: Misleading error message of the day Andrea Crotti <andrea.crotti.0@gmail.com> - 2011-12-08 14:39 +0000
  Re: Misleading error message of the day Chris Angelico <rosuav@gmail.com> - 2011-12-09 01:40 +1100
  Re: Misleading error message of the day Robert Kern <robert.kern@gmail.com> - 2011-12-08 14:47 +0000
    Re: Misleading error message of the day Roy Smith <roy@panix.com> - 2011-12-08 07:30 -0800
      Re: Misleading error message of the day Tim Chase <python.list@tim.thechases.com> - 2011-12-08 11:41 -0600
    Re: Misleading error message of the day Roy Smith <roy@panix.com> - 2011-12-08 07:30 -0800
  Re: Misleading error message of the day Jean-Michel Pichavant <jeanmichel@sequans.com> - 2011-12-08 16:03 +0100
    Re: Misleading error message of the day Roy Smith <roy@panix.com> - 2011-12-08 07:33 -0800
      Re: Misleading error message of the day Jean-Michel Pichavant <jeanmichel@sequans.com> - 2011-12-08 20:49 +0100
      Re: Misleading error message of the day Ethan Furman <ethan@stoneleaf.us> - 2011-12-08 12:13 -0800
      Re: Misleading error message of the day Lie Ryan <lie.1296@gmail.com> - 2011-12-09 12:46 +1100
        Re: Misleading error message of the day alex23 <wuwei23@gmail.com> - 2011-12-08 20:57 -0800
          Re: Misleading error message of the day Lie Ryan <lie.1296@gmail.com> - 2011-12-11 10:41 +1100
      Re: Misleading error message of the day Jean-Michel Pichavant <jeanmichel@sequans.com> - 2011-12-09 11:03 +0100
        Re: Misleading error message of the day Roy Smith <roy@panix.com> - 2011-12-09 09:43 -0500
      Re: Misleading error message of the day Ethan Furman <ethan@stoneleaf.us> - 2011-12-09 09:39 -0800
    Re: Misleading error message of the day Roy Smith <roy@panix.com> - 2011-12-08 07:33 -0800
      Re: Misleading error message of the day Grant Edwards <invalid@invalid.invalid> - 2011-12-08 18:10 +0000
        Re: Misleading error message of the day alister <alister.ware@ntlworld.com> - 2011-12-08 20:58 +0000
          Re: Misleading error message of the day Chris Angelico <rosuav@gmail.com> - 2011-12-09 08:17 +1100
        Re: Misleading error message of the day Roy Smith <roy@panix.com> - 2011-12-08 20:19 -0500
        Re: Misleading error message of the day Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-09 02:07 +0000
          Re: Misleading error message of the day Chris Angelico <rosuav@gmail.com> - 2011-12-09 14:46 +1100
            Re: Misleading error message of the day Roy Smith <roy@panix.com> - 2011-12-08 23:44 -0500
  Re: Misleading error message of the day Heiko Wundram <modelnine@modelnine.org> - 2011-12-08 16:16 +0100
    Re: Misleading error message of the day Roy Smith <roy@panix.com> - 2011-12-08 07:42 -0800
      Re: Misleading error message of the day Heiko Wundram <modelnine@modelnine.org> - 2011-12-08 16:50 +0100
        Re: Misleading error message of the day Roy Smith <roy@panix.com> - 2011-12-08 10:42 -0800
          Re: Misleading error message of the day Benjamin Kaplan <benjamin.kaplan@case.edu> - 2011-12-08 13:57 -0500
          Re: Misleading error message of the day Ethan Furman <ethan@stoneleaf.us> - 2011-12-08 11:09 -0800
          Re: Misleading error message of the day Benjamin Kaplan <benjamin.kaplan@case.edu> - 2011-12-08 14:38 -0500
          Re: Misleading error message of the day Ethan Furman <ethan@stoneleaf.us> - 2011-12-08 12:09 -0800
        Re: Misleading error message of the day Roy Smith <roy@panix.com> - 2011-12-08 10:42 -0800
      Re: Misleading error message of the day Andrea Crotti <andrea.crotti.0@gmail.com> - 2011-12-08 15:55 +0000
      Re: Misleading error message of the day Chris Angelico <rosuav@gmail.com> - 2011-12-09 03:21 +1100
      Re: Misleading error message of the day Robert Kern <robert.kern@gmail.com> - 2011-12-08 19:57 +0000
    Re: Misleading error message of the day Roy Smith <roy@panix.com> - 2011-12-08 07:42 -0800

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