[Python-Dev] PEP 418: Add monotonic clock

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