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

[Andrew Barnert]

On Mar 30, 2015, at 05:52, Joao S. O. Bueno <jsbueno at python.org.br> wrote:
> 
>> On 29 March 2015 at 19:01, Tim Delaney <timothy.c.delaney at gmail.com> wrote:
>>> 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.
> 
> If that is being done, anyway, I propose an optional parameter that
> would allow the context to internally use itertools.tee if the stream
> can't seek itself.

That sounds cool, but... If you have an actual file object with seek and tell, you're often using other methods besides iterating lines (like read), and tee won't help with those, so it might be a little misleading.

> Maybe with even three states:
> "strict":  raises an exception if the stream is not [back]seekable
> "default": creates a tee cache if the stream is not back-seekable
> "cache": always cache.
> 
> (And maybe a maxbuffer parameter to go along? )
> 
> 
> 
>> Tim Delaney
>> 
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