Queue qsize = unreliable?

[Peter Hansen]

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. """

-Peter