[Python-ideas] Add a context manager to keep stream position unchanged

[Tim Delaney]

On 30 March 2015 at 04:04, Dmitry Kazakov <jsbfox at gmail.com> wrote:

> I propose adding a context manager to contextlib module, which would
> restore the stream position of its argument at exit:
>
>     with contextlib.closing(file_like_object) as file:
>         file.read(1)
>         old = file.tell()
>         with contextlib.keep_stream_position(file) as f:
>             assert file is f
>             assert file_like_object is f
>             # Do something with the file(-like) object e.g. feed it
>             # to `PIL.Image.open` (will consume the whole stream)
>
>         assert file.tell() == old
>
> This CM would allow us to reuse streams efficiently, without requiring
> to call seek() explicitly. I will open a new issue and submit a patch
> in case of favorable responses.
>

I like the idea. Might need some bikeshedding on the name (maybe
restore_position?).

To test for seeking, I would suggest having a seek to current position
during construction. Not perfect (e.g. there may be streams that can only
seek forwards, not backwards) but I think it's the best non-destructive
option.

Tim Delaney
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