[Bug] TTLCache.expire(time) seems to be non-deterministic

[stephen-aruba]

Versions

Succeeds ✅
MacOS 10.15.7
Python 3.7, 3.8, 3.9
CacheTools v4.2.2

Unpredictable (results vary between runs) ✅❌
Ubuntu 20.04
Python 3.7, 3.8, 3.9
CacheTools v4.2.2

Reproduce

from datetime import datetime
from cachetools import TTLCache, cached

time_cache = TTLCache(maxsize=200, ttl=300)

@cached(time_cache)
def get_time(uuid: str) -> str:
    now = datetime.now()
    return now.strftime("%X.%f")

# return function response    
print(get_time("key"))    

# return cached result    
print(get_time("key"))  

# remove expired items from the cache, these should then be re-requested    
time_cache.expire(time=500)

# return function response    
print(get_time("key"))

Calling time_cache.expire(time=500) does not seem to expire items in the cache.

Prints:

<timestamp1>
<timestamp1>
<timestamp1>

Expected Behaviour

Calling time_cache.expire(time=500) should expire items in the cache, causing them to be re-requested.

Prints:

<timestamp1>
<timestamp1>
<timestamp2>  # new timestamp should be requested

Demo

Live demo (see Github Actions)

[tkem]

Why time_cache.expire(time=500)?

AFAICS you don't set a custom timer for your cache, so TTLCache uses time.monotonic() as it's timer function:
https://cachetools.readthedocs.io/en/stable/#cachetools.TTLCache

So you should pass the value of time.monotonic() at cache insertion time + specified ttl to time_cache.expire().

[tkem]

Better yet, since the timer is also a property of the cache

time_cache.expire(time_cache.timer() + 500)

[stephen-aruba]

Thanks for the suggestions, I think I misunderstood the operation of time.monotonic() on different platforms (on MacOS it seems to reset to zero at script initiation, I assumed this applied cross-platform).
I'll run your suggestions against my tests now and let you know...

[tkem]

Yeah, as you probably already found out, time.monotonic may count from any time point, so the actual value is pretty meaningless: https://docs.python.org/3/library/time.html#time.monotonic
I think it counts from last reboot (Linux uptime) on my machine, but I'm not quite sure, either.

[tkem]

However, extra kudos for your efforts, like creating a repo and actions for this - i I think this may be the best documented bug report I've received so far ;-)

[stephen-aruba]

Thanks very much for the suggestions. Both of them worked on Ubuntu and MacOS. I wan't able to verify for Windows (thus the failing check-marks above), and I suspect it would work on Windows too,but this solves my issues. Thank you 🙂

[stephen-aruba]

Thanks! Github actions was the easiest way to test cross-python and cross-platform on my side once I discovered behaviour differed between systems. Thank you for your quick response too - you've saved me some tech-debt on my side 🏅