Implement a simple LRUCache in Python

lru_cache.py

#!/usr/bin/env python
"""
Python's OrderedDict is an ideal data structure
to create an LRU cache
"""
from collections import OrderedDict


class LRUCache(object):

    def __init__(self, max_length=100000):
        self.cache = OrderedDict()
        self.max_length = max_length

    def __setitem__(self, key, value):
        if key in self.cache:
            self.cache.pop(key)
        self.cache[key] = value
        if len(self.cache) > self.max_length:
            self.cache.popitem(last=False)

    def __getitem__(self, key):
        if key in self.cache:
            value = self.cache.pop(key)
            self.cache[key] = value
            return value
        else:
            raise KeyError


def main():
    
    cache = LRUCache(3)

    cache[1] = 2
    cache[2] = 3
    cache[3] = 4
    cache[4] = 5
    cache[1] = 2
    
    assert cache[1] == 2
    try:
        cache[2]
    except KeyError:
        pass
    else:
        raise Exception('This should have thrown a KeyError')


if __name__ == '__main__':
    main()