__next__ and StopIteration

[Ian Kelly]

On Mon, Feb 9, 2015 at 5:59 PM, Chris Kaynor <ckaynor at zindagigames.com> wrote:
> On Mon, Feb 9, 2015 at 4:42 PM, Steven D'Aprano
> <steve+comp.lang.python at 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,

Fair point. I suppose that's because the file paradigm itself has a
concept of current position that can't easily be abstracted away.

> 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'

IIRC the docs also warn that mixing file iteration with other file
methods results in undefined or broken behavior. Maybe that's only for
Python 2 files, however.