Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #89759

Re: Python is not bad ;-)

Show key headers only | View raw

On Sat, May 2, 2015 at 8:22 PM, Dave Angel <davea@davea.name> wrote:
> On 05/02/2015 05:58 AM, Marko Rauhamaa wrote:
>>
>> Chris Angelico <rosuav@gmail.com>:
>>
>>> Guido is against anything that disrupts tracebacks, and optimizing
>>> tail recursion while maintaining traceback integrity is rather harder.
>>
>>
>> Tail recursion could be suppressed during debugging. Optimized code can
>> play all kinds of non-obvious tricks with the execution frame.
>>
>>> In the situations where it really is simple, you can always make the
>>> change in your own code anyway. Often, the process of converting
>>> recursion into tail recursion warps the code to the point where it's
>>> abusing recursion to implement iteration anyway, so just make it
>>> iterative.
>>
>>
>> While you shouldn't actively replace Python iteration with recursion, I
>> strongly disagree that naturally occurring tail recursion is abuse or
>> should be avoided in any manner.
>>
>
> When you strongly disagree, make sure you're disagreeing with what Chris
> actually said.  The key phrase in his message was
>     "converting recursion into tail recursion"
>
> NOT  "converting iteration into recursion"
> and NOT "naturally occurring tail recursion"

Indeed. I'll clarify my statement.

When a function needs to do further actions after the tail call, the
usual solution is to carry the information through parameters.

def stupid_sum_iter(x):
    """Calculate sum(range(x+1))"""
    tot = 0
    while x:
        tot += x
        x -= 1
    return tot

def stupid_sum_recur(x):
    return x and x + stupid_sum_recur(x - 1)

def stupid_sum_tail_recur(x, tot=0):
    if not x: return tot
    return stupid_sum_tail_recur(x - 1, tot + x)

Converting the recursive form into optimizable tail recursion requires
an accumulator parameter, which means it's virtually the same as the
iterative version. Naturally-occurring tail recursion does exist, but
it's far from all cases of recursion.

ChrisA

Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-04-30 09:07 +0200
  Re: Python is not bad ;-) Ben Finney <ben+python@benfinney.id.au> - 2015-04-30 19:10 +1000
    Re: Python is not bad ;-) Marko Rauhamaa <marko@pacujo.net> - 2015-04-30 13:16 +0300
      Re: Python is not bad ;-) Chris Angelico <rosuav@gmail.com> - 2015-04-30 20:52 +1000
      Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-04-30 13:30 +0200
        Re: Python is not bad ;-) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-05-01 17:03 +1000
          Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-05-01 09:47 +0200
          Re: Python is not bad ;-) Christian Gollwitzer <auriocus@gmx.de> - 2015-05-01 19:56 +0200
          Re: Python is not bad ;-) Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-05-02 19:44 +1200
          Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-05-02 10:26 +0200
            Re: Python is not bad ;-) Marko Rauhamaa <marko@pacujo.net> - 2015-05-02 12:10 +0300
              Re: Python is not bad ;-) Chris Angelico <rosuav@gmail.com> - 2015-05-02 19:25 +1000
                Re: Python is not bad ;-) Marko Rauhamaa <marko@pacujo.net> - 2015-05-02 12:58 +0300
                Re: Python is not bad ;-) Dave Angel <davea@davea.name> - 2015-05-02 06:22 -0400
                Re: Python is not bad ;-) Chris Angelico <rosuav@gmail.com> - 2015-05-02 20:42 +1000
                Re: Python is not bad ;-) Christian Gollwitzer <auriocus@gmx.de> - 2015-05-02 13:07 +0200
                Re: Python is not bad ;-) Chris Angelico <rosuav@gmail.com> - 2015-05-02 21:21 +1000
                Re: Python is not bad ;-) Christian Gollwitzer <auriocus@gmx.de> - 2015-05-02 13:32 +0200
                Re: Python is not bad ;-) Marko Rauhamaa <marko@pacujo.net> - 2015-05-02 14:42 +0300
                Re: Python is not bad ;-) Ian Kelly <ian.g.kelly@gmail.com> - 2015-05-02 09:45 -0600
                Re: Python is not bad ;-) Chris Angelico <rosuav@gmail.com> - 2015-05-03 01:55 +1000
                Re: Python is not bad ;-) Joonas Liik <liik.joonas@gmail.com> - 2015-05-02 16:50 +0300
                Re: Python is not bad ;-) Joonas Liik <liik.joonas@gmail.com> - 2015-05-02 18:53 +0300
                Re: Python is not bad ;-) Ian Kelly <ian.g.kelly@gmail.com> - 2015-05-02 11:00 -0600
                Re: Python is not bad ;-) Ian Kelly <ian.g.kelly@gmail.com> - 2015-05-02 11:17 -0600
                Re: Python is not bad ;-) Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-05-02 18:22 +0100
              Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-05-02 12:29 +0200
            Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-05-02 11:33 +0200
              Re: Python is not bad ;-) Dave Angel <davea@davea.name> - 2015-05-02 06:35 -0400
                Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-05-02 13:12 +0200
              Re: Python is not bad ;-) Ian Kelly <ian.g.kelly@gmail.com> - 2015-05-02 09:31 -0600
      Re: Python is not bad ;-) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-05-01 15:56 +1000
    Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-04-30 13:10 +0200
  Re: Python is not bad ;-) Michael Torrie <torriem@gmail.com> - 2015-04-30 08:03 -0600
    Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-04-30 18:11 +0200
      Re: Python is not bad ;-) Christian Gollwitzer <auriocus@gmx.de> - 2015-04-30 19:59 +0200
        Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-04-30 22:05 +0200

csiph-web