Queue qsize = unreliable?

Jacob Hallen jacob at cd.chalmers.se
Tue Aug 10 20:06:56 CEST 2004

In article <zeCdnfT6tL31Oo7cRVn-tg at powergate.ca>,
Peter Hansen  <peter at engcorp.com> wrote:
>James R. Saker Jr. wrote:
>> I see per pydoc that Queue.Queue()'s .qsize is allegedly unreliable:
>>      |  qsize(self)
>>      |      Return the approximate size of the queue (not reliable!).
>> Any thoughts on why this is unreliable (and more curiously, why it would
>> be put in there as an unreliable function?) Rather than roll my own
>> threaded fifo class, it would seem prudent to use Python's built-in
>> Queue but the warning signs on a rather necessary function seem curious.
>(Why do you think this function is necessary?  It's probably
>rare to really need it, except perhaps during debugging... )
>Anyway, the reason it's called "unreliable", though the term
>"inaccurate" might be more correct, is because while you are
>getting the size of the queue, it might be updated such that
>the new size is one or more fewer or larger than the value
>that is about to be returned to you.  In effect, the value is
>guaranteed accurate only for the precise instant in time, now
>passed, that it was determined, but by the time the calling
>routine actually sees the value the size could be anything.
>Note also the latest docs at docs.python.org, which state the
>case a little more clearly.
>"""Return the approximate size of the queue. Because of
>multithreading semantics, this number is not reliable. """

And an inaccurate number is still quite usable. If you are
the only one reading from a queue, you know that the number
is at least as large as indicated. If you read from multiple
queues, you can use the indicated number to pick which queue
to handle first.

This allows you to do pollling and fair scheduling and some other
neat tricks.

The same sort of tricks works for writers as well.

Jacob Hallén


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