[Python-Dev] threading.Semaphore()'s counter can become negative for non-ints (original) (raw)

Guido van Rossum guido at python.org
Mon Jan 30 19:52:29 CET 2012

TB, what's your use case for passing a float to a semaphore? Semaphores are conceptually tied to integers. You've kept arguing a few times now that the workaround you need are clumsy, but you've not explained why you're passing floats in the first place. A "fractional resource" just doesn't sound like a real use case to me.

On Mon, Jan 30, 2012 at 10:40 AM, T.B. <bauertomer at gmail.com> wrote:

On 2012-01-30 01:46, Victor Stinner wrote:

But why would you want to pass a float? It seems like API abuse to me. If something should be changed, Semaphore(arg) should raise a TypeError if arg is not an integer. Short version: I propose the the change to be -        while self.value == 0: +        while self.value < 1: This should not change the flow when Semaphore.value is an int. Longer explanation: I thought it is surprising to use math.floor() for threading.Semaphore, but now as you propose, we will need to use something like int(math.floor(value)) in Python2.x - which is even more surprising. That is because math.floor() (and round() for that matter) return a float object in Python2.x. Note: isinstance(4.0, numbers.Integral) is False, even in Python3.x, but until now 4.0 was valid as a value for Semaphore(). Also, using the builtin int()/math.trunc() on a float is probably not what you want here, but rather math.floor(). The value argument given to threading.Semaphore() is really a duck (or an object) that can be compared to 0 and 1, incremented by 1 and decremented by 1. These are properties that fit float. Why should you force the entire builtin int behavior on that object? I agree that using a float as the counter smells bad, but at times you might have something like a fractional resource (which is different from a floating point number). In such cases Semaphore.acquire(), after the tiny patch above, can be thought as checking if you have at least one "unit of resource" available. If you do have at least one such resource - acquire it. This will make sure the invariant "The counter can never go below zero" holds. Regards, TB

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-- --Guido van Rossum (python.org/~guido)

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