[Python-Dev] Small question about BufferedRandom spec (original) (raw)

Terry Reedy tjreedy at udel.edu
Tue Jan 6 05:13:14 CET 2009

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.

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