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Re: Nested iteration?

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From Peter Otten <__peter__@web.de>
Subject Re: Nested iteration?
Date Tue, 23 Apr 2013 18:15:42 +0200
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Xref csiph.com comp.lang.python:44195

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Roy Smith wrote:

> In reviewing somebody else's code today, I found the following
> construct (eliding some details):
> 
>     f = open(filename)
>     for line in f:
>         if re.search(pattern1, line):
>             outer_line = f.next()
>             for inner_line in f:
> if re.search(pattern2, inner_line):
>                     inner_line = f.next()
> 
> Somewhat to my surprise, the code worked.  I didn't know it was legal
> to do nested iterations over the same iterable (not to mention mixing
> calls to next() with for-loops).  Is this guaranteed to work in all
> situations?

That depends on what you mean by "all". A well-behaved iterator like 
Python's file object allows mixing of for loops and next(...) calls, but 
stupid people who deserve to burn in hell sometimes do

class MyIterable:
    def __iter__(self):
         reset_internal_counter()
         return self


with the consequence that every for loop implicitly resets the iterator's 
state.

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Thread

Nested iteration? roy@panix.com (Roy Smith) - 2013-04-23 11:40 -0400
  Re: Nested iteration? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 17:05 +0100
  Re: Nested iteration? Ian Kelly <ian.g.kelly@gmail.com> - 2013-04-23 10:05 -0600
  Re: Nested iteration? Peter Otten <__peter__@web.de> - 2013-04-23 18:15 +0200
  Re: Nested iteration? Chris Angelico <rosuav@gmail.com> - 2013-04-24 02:21 +1000
  Re: Nested iteration? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-04-23 16:35 +0000
  Re: Nested iteration? Ian Kelly <ian.g.kelly@gmail.com> - 2013-04-23 10:30 -0600
  Re: Nested iteration? Ian Kelly <ian.g.kelly@gmail.com> - 2013-04-23 10:39 -0600
  Re: Nested iteration? Chris Angelico <rosuav@gmail.com> - 2013-04-24 02:42 +1000
    Re: Nested iteration? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-04-23 16:53 +0000
  Re: Nested iteration? Terry Jan Reedy <tjreedy@udel.edu> - 2013-04-23 16:49 -0400
  Re: Nested iteration? Joshua Landau <joshua.landau.ws@gmail.com> - 2013-04-23 22:14 +0100
  Re: Nested iteration? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 22:29 +0100
  Re: Nested iteration? Joshua Landau <joshua.landau.ws@gmail.com> - 2013-04-23 22:41 +0100
  Re: Nested iteration? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 23:42 +0100

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