Recursion, memoization and mutable default arguments in Python

问题

"Base" meaning without just using lru_cache. All of these are "fast enough" -- I'm not looking for the fastest algorithm -- but the timings surprised me so I was hoping I could learn something about how Python "works".

Simple loop (/tail recursion):

def fibonacci(n):
    a, b = 0, 1
    if n in (a, b): return n
    for _ in range(n - 1):
        a, b = b, a + b
    return b

Simple memoized:

def fibonacci(n, memo={0:0, 1:1}):
    if len(memo) <= n:
        memo[n] = fibonacci(n - 1) + fibonacci(n - 2)
    return memo[n]

Using a generator:

def fib_seq():
    a, b = 0, 1
    yield a
    yield b
    while True:
        a, b = b, a + b
        yield b

def fibonacci(n):
    return next(x for (i, x) in enumerate(fib_seq()) if i == n)

I expected the first, being dead simple, to be the fastest. It's not. The second is by far the fastest, despite the recursion and lots of function calls. The third is cool, and uses "modern" features, but is even slower, which is disappointing. (I was tempted to think of generators as in some ways an alternative to memoization -- since they remember their state -- and since they're implemented in C I was hoping they'd be faster.)

Typical results:

loop: about 140 μs
memo: about 430 ns
genr: about 250 μs

So can anyone explain, in particular, why memoization is an order of magnitude faster than a simple loop?

EDIT:

Clear now that I have (like many before me) simply stumbled upon Python's mutable default arguments. This behavior explains the real and the apparent gains in execution speeds.

回答1:

What you're seeing is the whole point of memoization. The first time you call the function, the memo cache is empty and it has to recurse. But the next time you call it with the same or a lower parameter, the answer is already in the cache, so it returns immediately. if you perform thousands of calls, you're amortizing that first call's time over all the other calls. That's what makes memoization such a useful optimization, you only pay the cost the first time.

If you want to see how long it takes when the cache is fresh and you have to do all the recursions, you can pass the initial cache as an explicit argument in the benchmark call:

fibonacci(100, {0:0, 1:1})

来源：https://stackoverflow.com/questions/55068876/recursion-memoization-and-mutable-default-arguments-in-python

标签

python

performance

optimization

fibonacci

memoization