Time expiry dictionary in memory in Python

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I was just wondering how one might efficiently implement a time expiry dictionary in memory in Python such that key-value pairs expire after a specified time interval.

ttl caching dictionary python
1 Answers

The design pattern to typically do this is not via a dictionary, but via a function or method decorator. The dictionary is managed behind the scenes by the cache.

This answer uses the ttl_cache decorator in cachetools==3.1.0 with Python 3.7. It works a lot like functools.lru_cache, but with a time to live. As for its implementation logic, consider its source code.

import cachetools.func

@cachetools.func.ttl_cache(maxsize=128, ttl=10 * 60)
def example_function(key):
    return get_expensively_computed_value(key)


class ExampleClass:
    EXP = 2

    @classmethod
    @cachetools.func.ttl_cache()
    def example_classmethod(cls, i):
        return i**cls.EXP

    @staticmethod
    @cachetools.func.ttl_cache()
    def example_staticmethod(i):
        return i**3

If however you insist on using a dictionary, cachetools also has TTLCache.

import cachetools

ttl_cache = cachetools.TTLCache(maxsize=128, ttl=10 * 60)
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