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Re: Why Python 3?

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On 2014-04-20 17:22, Ian Kelly wrote:
>
> On Apr 19, 2014 2:54 PM, "Chris Angelico" <rosuav@gmail.com
> <mailto:rosuav@gmail.com>> wrote:
>  >
>  > On Sun, Apr 20, 2014 at 6:38 AM, Ian Kelly <ian.g.kelly@gmail.com
> <mailto:ian.g.kelly@gmail.com>> wrote:
>  > >> Or you just cast one of them to float. That way you're sure you're
>  > >> working with floats.
>  > >
>  > > Which is inappropriate if the type passed in was a Decimal or a
> complex.
>  >
>  > In that case, you already have a special case in your code, so whether
>  > that special case is handled by the language or by your code makes
>  > little difference. Is your function so generic that it has to be able
>  > to handle float, Decimal, or complex, and not care about the
>  > difference, and yet has to ensure that int divided by int doesn't
>  > yield int? Then say so; put in that special check. Personally, I've
>  > yet to meet any non-toy example of a function that needs that exact
>  > handling; most code doesn't ever think about complex numbers, and a
>  > lot of things look for one specific type:
>
> When I'm writing a generic average function, I probably don't know
> whether it will ever be used to average complex numbers. That shouldn't
> matter, because I should be able to rely on this code working for
> whatever numeric type I pass in:
>
> def average(values):
>      return sum(values) / len(values)
>
> This works for decimals, it works for fractions, it works for complex
> numbers, it works for numpy types, and in Python 3 it works for ints.
>
>  > Maybe it's not your code that should be caring about what happens when
>  > you divide two integers, but the calling code. If you're asking for
>  > the average of a list of numbers, and they're all integers, and the
>  > avg() function truncates to integer, then the solution is to use sum()
>  > and explicitly cast to floating point before dividing.
>
> First, that's not equivalent.  Try the following in Python 3:
>
> values = [int(sys.float_info.max / 10)] * 20
> print(average(values))
>
> Now try this:
>
> print(average(map(float, values)))
>
> I don't have an interpreter handy to test, but I expect the former to
> produce the correct result and the latter to raise OverflowError on the
> call to sum.
>

Python 3.4.0 (v3.4.0:04f714765c13, Mar 16 2014, 19:25:23) [MSC v.1600 64 
bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
 >>> import sys
 >>> def average(values):
...     return sum(values) / len(values)
...
 >>> values = [int(sys.float_info.max / 10)] * 20
 >>> print(average(values))
1.7976931348623158e+307
 >>> print(average(map(float, values)))
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File "<stdin>", line 2, in average
TypeError: object of type 'map' has no len()
 >>> print(average(list(map(float, values))))
inf
 >>>

In fact, that's true back to Python 3.1

> Second, why should the calling code have to worry about this
> implementation detail anyway? The point of a generic function is that
> it's generic.
>

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Thread

Why Python 3? Anthony Papillion <papillion@gmail.com> - 2014-04-18 22:28 -0500
  Re: Why Python 3? Paul Rubin <no.email@nospam.invalid> - 2014-04-18 23:40 -0700
    Re: Why Python 3? Chris Angelico <rosuav@gmail.com> - 2014-04-19 17:34 +1000
      Re: Why Python 3? Roy Smith <roy@panix.com> - 2014-04-19 09:26 -0400
        Re: Why Python 3? Chris Angelico <rosuav@gmail.com> - 2014-04-19 23:42 +1000
        Re: Why Python 3? Albert-Jan Roskam <fomcl@yahoo.com> - 2014-04-19 10:57 -0700
        Re: Why Python 3? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-04-20 10:07 +0000
    Re: Why Python 3? Ian Kelly <ian.g.kelly@gmail.com> - 2014-04-19 03:25 -0600
      Re: Why Python 3? Marko Rauhamaa <marko@pacujo.net> - 2014-04-19 12:59 +0300
    Re: Why Python 3? Chris Angelico <rosuav@gmail.com> - 2014-04-19 19:37 +1000
      Integer and float division [was Re: Why Python 3?] Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-04-20 11:02 +0000
        Re: Integer and float division [was Re: Why Python 3?] Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2014-04-20 15:38 +0300
          Re: Integer and float division [was Re: Why Python 3?] Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-04-20 15:09 +0000
        Re: Integer and float division [was Re: Why Python 3?] Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-04-21 11:44 +1200
    Re: Why Python 3? Terry Reedy <tjreedy@udel.edu> - 2014-04-19 13:23 -0400
      Re: Why Python 3? Paul Rubin <no.email@nospam.invalid> - 2014-04-19 20:25 -0700
        Re: Why Python 3? Ben Finney <ben+python@benfinney.id.au> - 2014-04-20 19:15 +1000
        Re: Why Python 3? Walter Hurry <walterhurry@lavabit.com> - 2014-04-20 23:50 +0000
          Re: Why Python 3? Chris Angelico <rosuav@gmail.com> - 2014-04-21 10:00 +1000
            Re: Why Python 3? HoneyMonster <nobody@someplace.invalid> - 2014-04-21 04:08 +0000
          Re: Why Python 3? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-04-21 01:11 +0100
    Re: Why Python 3? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-04-19 18:31 +0100
    Re: Why Python 3? Chris Angelico <rosuav@gmail.com> - 2014-04-20 03:53 +1000
    Re: Why Python 3? Ian Kelly <ian.g.kelly@gmail.com> - 2014-04-19 13:58 -0600
    Re: Why Python 3? Chris Angelico <rosuav@gmail.com> - 2014-04-20 06:31 +1000
      Re: Why Python 3? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-04-20 13:06 +1200
        Re: Why Python 3? Chris Angelico <rosuav@gmail.com> - 2014-04-20 11:28 +1000
          Re: Why Python 3? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-04-21 10:52 +1200
            Re: Why Python 3? Chris Angelico <rosuav@gmail.com> - 2014-04-21 09:24 +1000
              Re: Why Python 3? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-04-21 03:43 +0000
                Re: Why Python 3? Chris Angelico <rosuav@gmail.com> - 2014-04-21 14:43 +1000
                Re: Why Python 3? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-04-22 09:58 +1200
                Re: Why Python 3? Chris Angelico <rosuav@gmail.com> - 2014-04-21 14:48 +1000
                Re: Why Python 3? wxjmfauth@gmail.com - 2014-04-21 02:42 -0700
                Re: Why Python 3? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-04-22 10:28 +1200
                Re: Why Python 3? Chris Angelico <rosuav@gmail.com> - 2014-04-22 08:43 +1000
                Re: Why Python 3? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-04-22 18:03 +1200
        Re: Why Python 3? Terry Reedy <tjreedy@udel.edu> - 2014-04-20 00:17 -0400
          Re: Why Python 3? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-04-21 11:13 +1200
            Re: Why Python 3? Terry Reedy <tjreedy@udel.edu> - 2014-04-20 20:09 -0400
    Re: Why Python 3? Ian Kelly <ian.g.kelly@gmail.com> - 2014-04-19 14:38 -0600
    Re: Why Python 3? Chris Angelico <rosuav@gmail.com> - 2014-04-20 06:53 +1000
      Re: Why Python 3? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-04-20 13:35 +1200
    Re: Why Python 3? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-04-20 09:59 +0000
      Unicode in Python Rustom Mody <rustompmody@gmail.com> - 2014-04-21 20:57 -0700
        Re: Unicode in Python Terry Reedy <tjreedy@udel.edu> - 2014-04-22 01:44 -0400
          Re: Unicode in Python Rustom Mody <rustompmody@gmail.com> - 2014-04-21 23:18 -0700
            Re: Unicode in Python Chris Angelico <rosuav@gmail.com> - 2014-04-22 16:32 +1000
        Re: Unicode in Python Steven D'Aprano <steve@pearwood.info> - 2014-04-22 06:11 +0000
          Re: Unicode in Python Rustom Mody <rustompmody@gmail.com> - 2014-04-21 23:30 -0700
            Re: Unicode in Python Chris Angelico <rosuav@gmail.com> - 2014-04-22 16:44 +1000
            Re: Unicode in Python wxjmfauth@gmail.com - 2014-04-22 02:07 -0700
              Re: Unicode in Python Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-04-22 12:21 +0000
                Re: Unicode in Python wxjmfauth@gmail.com - 2014-04-22 08:28 -0700
        Re: Unicode in Python Ian Kelly <ian.g.kelly@gmail.com> - 2014-04-22 00:31 -0600
          Re: Unicode in Python Rustom Mody <rustompmody@gmail.com> - 2014-04-22 02:23 -0700
          Re: Unicode in Python Rustom Mody <rustompmody@gmail.com> - 2014-04-22 11:09 -0700
    Re: Why Python 3? Ian Kelly <ian.g.kelly@gmail.com> - 2014-04-20 10:22 -0600
      Re: Why Python 3? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-04-21 11:56 +1200
        Re: Why Python 3? Ian Kelly <ian.g.kelly@gmail.com> - 2014-04-20 18:29 -0600
    Re: Why Python 3? MRAB <python@mrabarnett.plus.com> - 2014-04-20 17:41 +0100
    Re: Why Python 3? Chris Angelico <rosuav@gmail.com> - 2014-04-21 02:46 +1000
      Re: Why Python 3? Roy Smith <roy@panix.com> - 2014-04-20 14:40 -0700
        Re: Why Python 3? Terry Reedy <tjreedy@udel.edu> - 2014-04-20 17:58 -0400
        Re: Why Python 3? Richard Damon <Richard@Damon-Family.org> - 2014-04-20 18:02 -0400
          Re: Why Python 3? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-04-21 12:22 +1200
        Re: Why Python 3? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-04-21 02:13 +0000
  Re: Why Python 3? Steve Hayes <hayesstw@telkomsa.net> - 2014-04-19 13:53 +0200
    Re: Why Python 3? Chris Angelico <rosuav@gmail.com> - 2014-04-19 22:46 +1000
    Re: Why Python 3? Rustom Mody <rustompmody@gmail.com> - 2014-04-19 08:59 -0700
  Re: Why Python 3? Rick Johnson <rantingrickjohnson@gmail.com> - 2014-04-19 07:41 -0700
  Re: Why Python 3? Thomas Lehmann <thomas.lehmann.private@googlemail.com> - 2014-05-06 09:28 -0700

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