bpo-44310: Add a FAQ entry for caching method calls

[rhettinger]

https://bugs.python.org/issue44310

[miss-islington]

Thanks @rhettinger for the PR 🌮🎉.. I'm working now to backport this PR to: 3.10, 3.9.
🐍🍒⛏🤖

[bedevere-bot]

GH-26777 is a backport of this pull request to the 3.10 branch.

[bedevere-bot]

GH-26778 is a backport of this pull request to the 3.9 branch.

[asottile-sentry]

pretty late to this but a hashable-mutable is usually a nono and the example is broken because of it:

from functools import lru_cache

class Weather:
    "Example with a mutable station identifier"

    def __init__(self, station_id):
        self.station_id = station_id

    def change_station(self, station_id):
        self.station_id = station_id

    def __eq__(self, other):
        return self.station_id == other.station_id

    def __hash__(self):
        return hash(self.station_id)

    @lru_cache(maxsize=20)
    def historic_rainfall(self, date, units='cm'):
        print('...recomputing')
        return f'Rainfall on a given date (for: {self.station_id}, {date}, {units})'


print("Weather(1)")
w1 = Weather(1)
print(w1.historic_rainfall('day1'))
w1.change_station(2)

print("Weather(1) again")
print(Weather(1).historic_rainfall('day1'))

output = '''\
$ python3 t3.py
Weather(1)
...recomputing
Rainfall on a given date (for: 1, day1, cm)
Weather(1) again
...recomputing
Rainfall on a given date (for: 1, day1, cm)
'''

not to mention the lifetime of the original Weather is extended indefinitely because it is stuck in the lru_cache