Extending TTLCache, expire() does not call popitem()

[mbrancato]

Before reporting a bug, please make sure you have the latest cachetools version installed:

pip install --upgrade cachetools

Describe the bug
For extending a class, the docs say all cache implementations call popitem() to evict items from the cache and indicate that overriding the popitem() can catch expiration from any class.

This does not work with TTLCache.

Expected result
Overriding popitem() should be called when an item is expired.

Actual result
The popitem() method is not called.

Reproduction steps

import time

from cachetools import TTLCache


class MyCache(TTLCache):
    def popitem(self):
        key, value = super().popitem()
        print('Key "%s" evicted with value "%s"' % (key, value))
        return key, value


c = MyCache(maxsize=20, ttl=2)
c["a"] = 1
time.sleep(1)
c["b"] = 2
time.sleep(4)
c["c"] = 3

[GianlucaFicarelli]

The report seems related to #205 and #103

If it's not possible or planned to add a notification callback when an item is removed because expired in TTLCache, it could be useful to make it more explicit in the docs.

[tkem]

Thanks @GianlucaFicarelli for pointing to the closed issues!

This use case seems to be more popular than I thought, so I might think about it again.
Maybe expire() returning a list of evicted keys, so you can override it in a derived class and act on the result of the base class method.
But don't expect anything too soon ;-)

But sure, the docs should definitely be improved in this respect.