Python - RecursionError is inconsistently thrown

Question

Why does Python throw a RecursionError on line 10 but not on line 13?

def fib(n, a = [0,1]):
    if len(a) > n:
        return a[n]
    a.append(fib(n - 1, a) + fib(n - 2, a))
    return a[n]


def main():
    x = 998
    print(fib(x)) # RecursionError

    for i in range(1000):
        print(fib(i)) # No Error

main()

Answer 1

Looking further into this, it looks like the previously mentioned mutable default argument treatment by python is actually the reason behind this. Instead of creating a new list every time the function is called, the list is created once when the function is defined.

Here is a function exploiting this feature, so that calling fib(998) doesn't throw the RecursionError:

def fib(n, a = [0,1]):
    if len(a) > n:
        return a[n]
    while len(a) <= n:
        a.append(fib(len(a) - 1) + fib(len(a) - 2))
    return a[n]