Jeremy Hylton wrote:
>>>Are the thread semantics for file objecst documented anywhere? I
>>>don't see anything in the library manual, which is where I expected to
>>>find it. It looks like read and write are atomic by virtue of fread
>>>and fwrite being atomic.
>>
>>Uncle Timmy will no doubt agree with me: the semantics don't matter.
>>NEVER, NEVER access the same file object from multiple threads, unless
>>you're using a lock. And even using a lock is stupid.
>
>
> I'm not looking for your permission or approval.
Literally, the answer to your question is "no". In fact, Python does not
specify *any* interleaving semantics for threads whatsoever. The only
statement to this respect is
"""
Not all built-in functions that may block waiting for I/O allow other
threads to run. (The most popular ones (\function{time.sleep()},
\method{\var{file}.read()}, \function{select.select()}) work as
expected.)
"""
Of course, this says it works as expected, without saying what actually
is expected.
> I just want to know what semantics are intended.
But this is not what you've asked :-)
Anyway, expected by whom? Aahz clearly expects that the semantics are
unspecified, as he expects that nobody ever even attempts to read the
same file from multiple threads.
> If the documentation wants to say that
> the semantics are undefined that okay,
Formally, there is no need to say that something is undefined. Not
defining anything is sufficient. So the semantics *is* undefined,
whether the documentation "wants" to say that or not.
> although I think we need to say
> more because some behavior has been provided by the implementation for
> a long time.
That immediately rings the Jython bell, and perhaps also the PyPy
bell.
So if you want to say something, just go ahead. Before I make the
documentation want to say that, I would like to make it say more
basic things first (e.g. that stores to variables are atomic).
Regards,
Martin
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