precursion

precursion – Python module to avoid RecursionError: maximum recursion depth exceeded easily

Usage

Ok, let's write recursive function:

def sumrange(x):
    if x == 0:
        return 0

    r = sumrange(x - 1)
    return x + r

print(sumrange(10)  # 55

Pretty simple. But what if we call it with bigger argument

print(sumrange(1000))
# RecursionError: maximum recursion depth exceeded

Let's fix it with precursion module:

@precurse
def sumrange(x):
    if x == 0:
        # return was:
        # return 0
        # now we need to use StopIteration exception:
        raise StopIteration(0)

    # recursive call was:
    # r = sumrange(x - 1)
    # now we use yield:
    r = yield sumrange.r(x - 1)
    raise StopIteration(x + r)

print(sumrange(1000))  # 500500!!1

That's it!

What is .r in sumrange.r?

It's unwrapped function, so you yield unwrapped generator

Pros and cons:

Pros

The code looks cleaner. Yep.

Cons

Function calls have performance and memory overhead, so using the decorator is slower than if you use Tail-call optimization via while or implement a stack inside a function.