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Re: how to avoid checking the same condition repeatedly ?

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On Monday, October 28, 2013 6:50:19 AM UTC-3, Wolfgang Maier wrote:
> Dear all,
> 
> this is a recurring programming problem that I'm just not sure how to solve
> 
> optimally, so I thought I'd ask for your advice:
> 
> imagine you have a flag set somewhere earlier in your code, e.g.,
> 
> 
> 
> needs_processing = True
> 
> 
> 
> then in a for loop you're processing the elements of an iterable, but the
> 
> kind of processing depends on the flag, e.g.,:
> 
> 
> 
> for elem in iterable:
> 
>     if needs_processing:
> 
>         pre_process(elem)  # reformat elem in place
> 
>     print(elem)
> 
> 
> 
> this checks the condition every time through the for loop, even though there
> 
> is no chance for needs_processing to change inside the loop, which does not
> 
> look very efficient. Of course, you could rewrite the above as:
> 
> 
> 
> if needs_processing:
> 
>     for elem in iterable:
> 
>         pre_process(elem)  # reformat elem in place
> 
>         print(elem)
> 
> else:
> 
>     for elem in iterable:
> 
>         print(elem)
> 
> 
> 
> but this means unnecessary code-duplication.
> 
> 
> 
> You could also define functions (or class methods):
> 
> def pre_process_and_print (item):
> 
>     pre_process(item)
> 
>     print(item)
> 
> 
> 
> def raw_print (item):
> 
>     print(item)
> 
> 
> 
> then:
> 
> process = pre_process_and_print if needs_processing else raw_print
> 
> for elem in iterable:
> 
>     process(elem)
> 
> 
> 
> but while this works for the simple example here, it becomes complicated if
> 
> pre_process requires more information to do its job because then you will
> 
> have to start passing around (potentially lots of) arguments.
> 
> 
> 
> So my question is: is there an agreed-upon generally best way of dealing
> 
> with this?
> 
> 
> 
> Thanks for your help,
> 
> Wolfgang

Hi All, 
Trying to help you out, I wonder if something like the following code will help on what you need.

for elem in (needs_processing and iterable or []):
    pre_process(elem)  # reformat elem in place
    print(elem) 

It avoids code duplication and only process and iterates if the condition is True.
I hope it helps, otherwise, we could keep thinking alternatives.

Regards.
Mariano.

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how to avoid checking the same condition repeatedly ? Wolfgang Maier <wolfgang.maier@biologie.uni-freiburg.de> - 2013-10-28 09:50 +0000
  Re: how to avoid checking the same condition repeatedly ? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-10-28 11:14 +0000
    Re: how to avoid checking the same condition repeatedly ? Chris Angelico <rosuav@gmail.com> - 2013-10-28 22:18 +1100
      Re: how to avoid checking the same condition repeatedly ? Steven D'Aprano <steve@pearwood.info> - 2013-10-29 04:51 +0000
  Re: how to avoid checking the same condition repeatedly ? Piet van Oostrum <piet@vanoostrum.org> - 2013-10-28 14:31 -0400
  Re: how to avoid checking the same condition repeatedly ? Nobody <nobody@nowhere.com> - 2013-10-28 19:37 +0000
    Re: how to avoid checking the same condition repeatedly ? Neil Cerutti <neilc@norwich.edu> - 2013-10-29 13:44 +0000
      Re: how to avoid checking the same condition repeatedly ? rusi <rustompmody@gmail.com> - 2013-10-29 09:13 -0700
  Re: how to avoid checking the same condition repeatedly ? Peter Cacioppi <peter.cacioppi@gmail.com> - 2013-10-28 23:14 -0700
    Re: how to avoid checking the same condition repeatedly ? Chris Angelico <rosuav@gmail.com> - 2013-10-29 17:39 +1100
  Re: how to avoid checking the same condition repeatedly ? Peter Cacioppi <peter.cacioppi@gmail.com> - 2013-10-28 23:53 -0700
    Re: how to avoid checking the same condition repeatedly ? Chris Angelico <rosuav@gmail.com> - 2013-10-29 18:09 +1100
  Re: how to avoid checking the same condition repeatedly ? Peter Cacioppi <peter.cacioppi@gmail.com> - 2013-10-29 16:55 -0700
  Re: how to avoid checking the same condition repeatedly ? alex23 <wuwei23@gmail.com> - 2013-10-30 11:38 +1000
  Re: how to avoid checking the same condition repeatedly ? Mariano Anaya <marianoanaya@gmail.com> - 2013-10-30 06:34 -0700

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