[Python-Dev] cpython: Issue #10278: Add an optional strict argument to time.steady(), False by default (original) (raw)

Steven D'Aprano steve at pearwood.info
Tue Mar 20 07:54:57 CET 2012

On Mon, Mar 19, 2012 at 01:35:49PM +0100, Victor Stinner wrote:

Said differently: time.steady(strict=True) is always monotonic (*), whereas time.steady() may or may not be monotonic, depending on what is avaiable.

time.steady() is a best-effort steady clock. (*) time.steady(strict=True) relies on the OS monotonic clock. If the OS provides a "not really monotonic" clock, Python cannot do better.

I don't think that is true. Surely Python can guarantee that the clock will never go backwards by caching the last value. A sketch of an implementation:

def monotonic(_last=[None]): t = system_clock() # best effort, but sometimes goes backwards if _last[0] is not None: t = max(t, _last[0]) _last[0] = t return t

Overhead if done in Python may be excessive, in which case do it in C.

Unless I've missed something, that guarantees monotonicity -- it may not advance from one call to the next, but it will never go backwards.

There's probably even a cleverer implementation that will not repeat the same value more than twice in a row. I leave that as an exercise :)

As far as I can tell, "steady" is a misnomer. We can't guarantee that the timer will tick at a steady rate. That will depend on the quality of the hardware clock.

-- Steven

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