Re: Feature suggestion -- return if true

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Date	Sun, 17 Apr 2011 19:07:03 +1000
Subject	Re: Feature suggestion -- return if true
From	Chris Angelico <rosuav@gmail.com>
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On Sun, Apr 17, 2011 at 6:45 PM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> On Sun, 17 Apr 2011 16:21:53 +1200, Gregory Ewing wrote:
>
>> Chris Angelico wrote:
>>
>>> def fac(n):
>>>     # attempt to get from a cache
>>>     return? cache[n]
>>>     # not in cache, calculate the value
>>>     ret=1 if n<=1 else fac(n-1)*n
>>>     # and cache and return it
>>>     cache[n]=ret; return ret
>>
>> My idiom for fetching from a cache looks like this:
>>
>>    def get_from_cache(x):
>>      y = cache.get(x)
>>      if not y:
>>        y = compute_from(x)
>>        cache[x] = y
>>      return y
>>
>> which doesn't require any conditional returns.
>
>
> I'm sure you realise that that snippet needlessly recalculates any cached
> result that happens to be false, but others reading might not.

Sure. In my (somewhat contrived) example of factorials, that's going
to be true (apart from 0! = 0); and if the function returns a string
or other object rather than an integer, same thing. If there's the
possibility of _ANY_ value coming back from the computation, then it
would need to be done as:

if x in cache: return cache[x]

or as:

try:
  return cache[x]
except KeyError:
  # calculate etc

Obviously, as with everything, you need to know your own code and your own data.

Chris Angelico

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Feature suggestion -- return if true zildjohn01 <zildjohn01@gmail.com> - 2011-04-11 16:17 -0700
  Re: Feature suggestion -- return if true James Mills <prologic@shortcircuit.net.au> - 2011-04-12 10:27 +1000
    Re: Feature suggestion -- return if true Grant Edwards <invalid@invalid.invalid> - 2011-04-12 01:44 +0000
      Re: Feature suggestion -- return if true James Mills <prologic@shortcircuit.net.au> - 2011-04-12 12:12 +1000
        Re: Feature suggestion -- return if true Grant Edwards <invalid@invalid.invalid> - 2011-04-12 02:18 +0000
          Re: Feature suggestion -- return if true James Mills <prologic@shortcircuit.net.au> - 2011-04-12 12:44 +1000
            Re: Feature suggestion -- return if true Grant Edwards <invalid@invalid.invalid> - 2011-04-12 13:42 +0000
            Re: Feature suggestion -- return if true "Martin v. Loewis" <martin@v.loewis.de> - 2011-04-17 12:03 +0200
              Re: Feature suggestion -- return if true James Mills <prologic@shortcircuit.net.au> - 2011-04-18 09:36 +1000
      Re: Feature suggestion -- return if true James Mills <prologic@shortcircuit.net.au> - 2011-04-12 12:20 +1000
      Re: Feature suggestion -- return if true Zero Piraeus <schesis@gmail.com> - 2011-04-11 22:43 -0400
      Re: Feature suggestion -- return if true Chris Angelico <rosuav@gmail.com> - 2011-04-12 12:44 +1000
      Re: Feature suggestion -- return if true Chris Angelico <rosuav@gmail.com> - 2011-04-12 12:49 +1000
      Re: Feature suggestion -- return if true James Mills <prologic@shortcircuit.net.au> - 2011-04-12 12:59 +1000
      Re: Feature suggestion -- return if true James Mills <prologic@shortcircuit.net.au> - 2011-04-12 13:01 +1000
        Re: Feature suggestion -- return if true Nobody <nobody@nowhere.com> - 2011-04-12 07:08 +0100
          Re: Feature suggestion -- return if true James Mills <prologic@shortcircuit.net.au> - 2011-04-12 16:21 +1000
            Re: Feature suggestion -- return if true Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-04-12 10:52 +0000
            Re: Feature suggestion -- return if true scattered <tooscattered@gmail.com> - 2011-04-12 03:55 -0700
              Re: Feature suggestion -- return if true "Colin J. Williams" <cjw@ncf.ca> - 2011-04-12 10:01 -0400
            Re: Feature suggestion -- return if true Grant Edwards <invalid@invalid.invalid> - 2011-04-12 13:50 +0000
        Re: Feature suggestion -- return if true Grant Edwards <invalid@invalid.invalid> - 2011-04-12 13:44 +0000
      Re: Feature suggestion -- return if true Westley Martínez <anikom15@gmail.com> - 2011-04-12 07:05 -0700
        Re: Feature suggestion -- return if true scattered <tooscattered@gmail.com> - 2011-04-12 07:58 -0700
          Re: Feature suggestion -- return if true Westley Martínez <anikom15@gmail.com> - 2011-04-12 15:45 -0700
          Re: Feature suggestion -- return if true Chris Angelico <rosuav@gmail.com> - 2011-04-13 08:52 +1000
        Re: Feature suggestion -- return if true zildjohn01 <zildjohn01@gmail.com> - 2011-04-12 11:25 -0700
          Re: Feature suggestion -- return if true Terry Reedy <tjreedy@udel.edu> - 2011-04-12 15:14 -0400
          Re: Feature suggestion -- return if true alex23 <wuwei23@gmail.com> - 2011-04-12 21:05 -0700
    Re: Feature suggestion -- return if true Teemu Likonen <tlikonen@iki.fi> - 2011-04-12 21:00 +0300
      Re: Feature suggestion -- return if true Ian Kelly <ian.g.kelly@gmail.com> - 2011-04-12 12:25 -0600
        Re: Feature suggestion -- return if true Neil Cerutti <neilc@norwich.edu> - 2011-04-12 20:13 +0000
          Re: Feature suggestion -- return if true Neil Cerutti <neilc@norwich.edu> - 2011-04-12 20:16 +0000
      Re: Feature suggestion -- return if true Ian Kelly <ian.g.kelly@gmail.com> - 2011-04-12 12:29 -0600
      Re: Feature suggestion -- return if true Paul Rudin <paul.nospam@rudin.co.uk> - 2011-04-12 19:28 +0100
      Re: Feature suggestion -- return if true Chris Rebert <clp2@rebertia.com> - 2011-04-12 13:26 -0700
      Re: Feature suggestion -- return if true James Mills <prologic@shortcircuit.net.au> - 2011-04-13 08:34 +1000
      Re: Feature suggestion -- return if true Ethan Furman <ethan@stoneleaf.us> - 2011-04-12 16:06 -0700
        Re: Feature suggestion -- return if true Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2011-04-21 12:13 +0200
      Re: Feature suggestion -- return if true James Mills <prologic@shortcircuit.net.au> - 2011-04-13 09:12 +1000
      Re: Feature suggestion -- return if true Westley Martínez <anikom15@gmail.com> - 2011-04-12 16:15 -0700
      Re: Feature suggestion -- return if true Ethan Furman <ethan@stoneleaf.us> - 2011-04-12 16:48 -0700
        Re: Feature suggestion -- return if true Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-04-13 00:03 +0000
          Re: Feature suggestion -- return if true Ethan Furman <ethan@stoneleaf.us> - 2011-04-12 19:42 -0700
          Re: Feature suggestion -- return if true Chris Angelico <rosuav@gmail.com> - 2011-04-13 13:00 +1000
          Re: Feature suggestion -- return if true James Mills <prologic@shortcircuit.net.au> - 2011-04-13 13:28 +1000
      Re: Feature suggestion -- return if true Teemu Likonen <tlikonen@iki.fi> - 2011-04-13 13:58 +0300
  Re: Feature suggestion -- return if true Chris Angelico <rosuav@gmail.com> - 2011-04-12 10:46 +1000
    Re: Feature suggestion -- return if true Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2011-04-17 16:21 +1200
      Re: Feature suggestion -- return if true Chris Angelico <rosuav@gmail.com> - 2011-04-17 14:31 +1000
      Re: Feature suggestion -- return if true Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-04-17 08:45 +0000
        Re: Feature suggestion -- return if true Chris Angelico <rosuav@gmail.com> - 2011-04-17 19:07 +1000
          Re: Feature suggestion -- return if true Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-04-17 15:23 +0000
        Re: Feature suggestion -- return if true Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2011-04-18 12:25 +1200
        Re: Feature suggestion -- return if true Dave Angel <davea@ieee.org> - 2011-04-17 22:04 -0400
        Re: Feature suggestion -- return if true Chris Angelico <rosuav@gmail.com> - 2011-04-18 12:10 +1000
          Re: Feature suggestion -- return if true Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2011-04-19 12:35 +1200
            Re: Feature suggestion -- return if true Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-04-19 09:42 +0300
              Re: Feature suggestion -- return if true Chris Angelico <rosuav@gmail.com> - 2011-04-19 17:01 +1000
      Re: Feature suggestion -- return if true "D'Arcy J.M. Cain" <darcy@druid.net> - 2011-04-17 08:33 -0400
        Re: Feature suggestion -- return if true Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-04-17 15:10 +0000
        Re: Feature suggestion -- return if true aahz@pythoncraft.com (Aahz) - 2011-04-18 09:11 -0700
      Re: Feature suggestion -- return if true Greg Ewing <greg.ewing@canterbury.ac.nz> - 2011-04-18 11:09 +1200
  Re: Feature suggestion -- return if true Chris Angelico <rosuav@gmail.com> - 2011-04-12 10:51 +1000
  Re: Feature suggestion -- return if true Paul Rubin <no.email@nospam.invalid> - 2011-04-11 23:58 -0700
    Re: Feature suggestion -- return if true John Roth <johnroth1@gmail.com> - 2011-04-12 06:16 -0700
    Re: Feature suggestion -- return if true Mel <mwilson@the-wire.com> - 2011-04-12 11:12 -0400

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