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Re: __next__ and StopIteration

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On Mon, Feb 9, 2015 at 4:42 PM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> so that's an excellent sign that doing so is best practice, but it should
> not be seen as *required*. After all, perhaps you have good reason for
> wanting your iterable class to only be iterated over once.

In fact, there is one in the stdlib, the "file" object, which has a
__iter__ which returns self. The code below shows this,

There are a number of good reasons: perhaps there is no good way to
reset the iterator or there is outside state that has to be managed.
I'm also sure there are other reasons I cannot think of right now.

>
> Also, *technically* iterators may be re-iterable. The docs say that
> iterators which fail to raise StopIteration forever once they are exhausted
> are "broken", but the docs do not forbid broken iterators. Consenting
> adults and all that. You might want an iterator with a reset() method. Even
> an outright broken iterator!
>
>     def __next__(self):
>         if random.random() < 0.1: raise StopIteration
>         return random.random()
>
> Why you would want one, I don't know, but if you have a hankering for such a
> beast, Python lets you do it.

The "file" object is also an example of this. It is technically a
broken iterator according to the docs:

Python 3.4.2 (v3.4.2:ab2c023a9432, Oct  6 2014, 22:15:05) [MSC v.1600
32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> f = open("d:/test.txt")
>>> iter(f) is f
True
>>> for l in f:
...     print(l)
...
line 1

line 2

line 3

>>> for l in f:
...     print(l)
...
>>> f.seek(0)
0
>>> for l in f:
...     print(l)
...
line 1

line 2

line 3

>>> next(f)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration
>>> f.seek(0)
0
>>> next(f) # This should throw StopIteration as it has previously thrown StopIteration.
'line 1\n'
>>>



Chris

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__next__ and StopIteration Charles Hixson <charleshixsn@earthlink.net> - 2015-02-09 11:14 -0800
  Re: __next__ and StopIteration Rob Gaddi <rgaddi@technologyhighland.invalid> - 2015-02-09 19:27 +0000
  Re: __next__ and StopIteration Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-02-10 10:30 +1100
    Re: __next__ and StopIteration Ian Kelly <ian.g.kelly@gmail.com> - 2015-02-09 16:56 -0700
      Re: __next__ and StopIteration Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-02-10 11:42 +1100
        Re: __next__ and StopIteration Chris Angelico <rosuav@gmail.com> - 2015-02-10 11:54 +1100
          Re: __next__ and StopIteration Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-02-10 12:11 +1100
            Re: __next__ and StopIteration Chris Angelico <rosuav@gmail.com> - 2015-02-10 12:58 +1100
        Re: __next__ and StopIteration Chris Kaynor <ckaynor@zindagigames.com> - 2015-02-09 16:59 -0800
          Re: __next__ and StopIteration Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-02-10 16:54 +1100
        Re: __next__ and StopIteration Ian Kelly <ian.g.kelly@gmail.com> - 2015-02-10 01:30 -0700
        Re: __next__ and StopIteration Chris Kaynor <ckaynor@zindagigames.com> - 2015-02-10 09:27 -0800
    Re: __next__ and StopIteration Charles Hixson <charleshixsn@earthlink.net> - 2015-02-09 20:33 -0800
    Re: __next__ and StopIteration Chris Angelico <rosuav@gmail.com> - 2015-02-10 15:46 +1100
    Re: __next__ and StopIteration Charles Hixson <charleshixsn@earthlink.net> - 2015-02-09 22:16 -0800
    Re: __next__ and StopIteration Chris Angelico <rosuav@gmail.com> - 2015-02-10 17:38 +1100
    Re: __next__ and StopIteration Ethan Furman <ethan@stoneleaf.us> - 2015-02-10 08:44 -0800
    Re: __next__ and StopIteration Ian Kelly <ian.g.kelly@gmail.com> - 2015-02-10 09:53 -0700
    Re: __next__ and StopIteration Ethan Furman <ethan@stoneleaf.us> - 2015-02-10 09:33 -0800

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