lru_cache

lru_cache from the functools module is an example of a decorator and an example of memoization.

lru_cache memoizes the inputs and outputs of the decorated function in a size-restricted dictionary. It speeds up repeated calls to a slow function with the same inputs. For instance, if the function reads from disk, makes network requests, or requires a lot of computation AND it is used repeatedly with the same inputs.

Here's an example from the Python documentation that perfectly illustrates how and why to use the lru_cache decorator:

from functools import lru_cache

@lru_cache()
def factorial_r(x):
    if x == 0:
        return 1
    else:
        return x * factorial_r(x - 1)

factorial_r(10) # no previously cached result, makes 11 recursive calls
# 3628800
factorial_r(5)  # just looks up cached value result
# 120
factorial_r(12) # makes two new recursive calls, the other 11 are cached
# 479001600

Since the factorial function is recursive and the inputs are sequential numbers, it's called repeatedly with the same inputs. Without the cache, the function would be called 30 times. With lru_cache, the function is only called 13 times. While you don't often need to compute factorials, this example ties together how to use a decorator and memoization and recursion.

Assignment

The creator of Doc2Doc is a huge fan of palindromes for some nerdy reason. Add a feature to check if a word is a palindrome.

Import the lru_cache function from the functools module and use it to decorate the incomplete is_palindrome function.
Complete the is_palindrome function. It takes as input a word string and returns True if the word is a palindrome (such as "racecar"), or False otherwise.

Try to use recursion. Check the outer characters first, then move inwards until you reach the base case or find the word is not a palindrome.