[Python-Dev] Small question about BufferedRandom spec

[Terry Reedy]

Guido van Rossum wrote:

>> « Q: Do we want to mandate in the specification that switching between reading
>> and writing on a read-write object implies a .flush()? Or is that an
>> implementation convenience that users should not rely on? »
>>
>> Is it ok if I assume that the answer is "it is an implementation convenience
>> that users should not rely on"? The reason is that I'm overhauling
>> BufferedRandom objects to use a single shared buffer, so as to optimize
>> interleaved reads and writes.
> 
> I think it's fine if the flush to the file is optional, as long as
> this is clearly documented. However, the semantics of interleaving
> reads and writes, with and without seek calls in between, should be
> well-defined and correct/useful, so that it behaves the same
> regardless of the buffer size.

I don't know how much of the stdio will be wrapped or replaced, but, 
FWIW, the C89 Standard, as described by Plauger & Brodie, requires a 
position-setting operation between writes and reads: one of fflush, 
fseek, fsetpos, or rewind.  Same for reads and writes unless the read 
set EOF.

> 
> Ditto for the flush call currently implied by a seek -- if you can
> satisfy the seek by moving where you are in the buffer without
> flushing, that's fine IMO, but it should be well documented.
> 
> It should also be documented that a flush still *may* occur, of course.
>