I wrote the program below in Python for question 509 on LeetCode, which asks the user to write a program to find the value of the nth number in the Fibonacci sequence:

class Solution(object):
    def fib(self, n):
        output = 0
        if n == 0:
            output = 0
        elif n == 1:
            output = 1
        else:
            output = fib(n-1) + fib(n-2)
        return(output)

I was then met with this error:

NameError: global name 'fib' is not defined
    output = fib(n-1) + fib(n-2)
Line 9 in fib (Solution.py)
    ret = Solution().fib(param_1)
Line 30 in _driver (Solution.py)
    _driver()
Line 40 in <module> (Solution.py)

I then took a look at all the solutions posted and I noticed that all the solutions that utilized linear recursion had one thing in common: They all used self. as a prefix before calling the fib() function in the body of the initial function. As such, I changed my solution to:

class Solution(object):
    def fib(self, n):
        output = 0
        if n == 0:
            output = 0
        elif n == 1:
            output = 1
        else:
            output = self.fib(n-1) + self.fib(n-2)
        return(output)

The error was gone and LeetCode accepted my submission.

The weird thing is that I ran the initial code without self. in an IDE, and there was no runtime error; the program was working.

Why did I have to use .self in order to get rid of the runtime error on LeetCode?