Fix TLRUCache keeping stale value when overwriting a key with an expired value

[gaoflow]

Assigning an already-expired value to an existing key in a TLRUCache is silently swallowed: __setitem__ returns early before storing the new value, but it leaves the previous entry in place. The cache then keeps returning the old value, even though the assignment should have replaced (or removed) it.

from cachetools import TLRUCache
c = TLRUCache(maxsize=10, ttu=lambda k, v, now: now + v, timer=lambda: 0.0)
c['x'] = 5
c['x'] = 0          # ttu -> now, i.e. immediately expired
c['x']              # -> 5  (stale old value; expected: KeyError / absent)
'x' in c            # -> True

When the new value is dead-on-arrival, drop any existing entry for that key so it is evicted instead of retaining the stale value. New-key inserts of expired values remain a no-op as before. Added a regression test.

[tkem]

Wow, nice catch! Thanks for reporting, and especially for providing a PR including sound regression tests!
However, I think it could be argued both ways - I wouldn't necessarily consider an "old" value that's not expired "stale", in this case.
So if the current value has still some time left, trying to replace it with something "dead-on-arrival" (I like that term very much!) may as well leave the old item in place, as far as I'm concerned. IIRC, this is also the approach with caches that provide a getsize parameter - trying to set a key/value pair that by itself exceeds the maximum cache size will be rejected immediately, but will leave an existing entry with the same key intact. So maybe that's existing practice which does support the way it's currently implemented, but I'll have to think a bit about it.
Anyway, thanks for bringing this up and providing a high-quality PR!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (48284d7) to head (9ea6948).

Additional details and impacted files

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##            master      #400   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            5         5           
  Lines         1084      1089    +5     
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+ Hits          1084      1089    +5     

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