[Python-Dev] PEP 418: Add monotonic clock (original) (raw)

Steven D'Aprano steve at pearwood.info
Wed Mar 28 19:00:13 CEST 2012

Nick Coghlan wrote:

Completely unintuitive and unnecessary. With the GIL taking care of synchronisation issues, we can easily coerce time.time() into being a monotonic clock by the simple expedient of saving the last returned value: [snip]

Here's a version that doesn't suffer from the flaw of returning a long stream of constant values when the system clock jumps backwards a significant amount:

class MockTime: def init(self): self.ticks = [1, 2, 3, 4, 2, 3, 4, 5, 7, 3, 4, 6, 7, 8, 8, 9] self.i = -1 def call(self): self.i += 1 return self.ticks[self.i]

time = MockTime()

_prev = _prev_raw = 0 def monotonic(): global _prev, _prev_raw raw = time() delta = max(0, raw - _prev_raw) _prev_raw = raw _prev += delta return _prev

And in use:

[monotonic() for i in range(16)] [1, 2, 3, 4, 4, 5, 6, 7, 9, 9, 10, 12, 13, 14, 14, 15]

Time: [1, 2, 3, 4, 2, 3, 4, 5, 7, 3, 4, 6, 7, 8, 8, 9] Nick: [1, 2, 3, 4, 4, 4, 4, 5, 7, 7, 7, 7, 7, 8, 8, 9] Mine: [1, 2, 3, 4, 4, 5, 6, 7, 9, 9, 10, 12, 13, 14, 14, 15]

Mine will get ahead of the system clock each time it jumps back, but it's a lot closer to the ideal of a strictly monotonically increasing clock.

Assuming that the system clock will never jump backwards twice in a row, the double-caching version will never have more than two constant values in a row.

-- Steven

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