def fibonacci(n):
    if ( n < 0 ):  # Termination condition
        print("Invalid input")
        return
    elif ( n==0 ):  # Termination condition
        return 0  # First Fibonacci number is 0
    elif ( n==1 ):  # Termination condition
        return 1  # Second Fibonacci number is 1
    else:
        return fibonacci(n-1) + fibonacci(n-2)  # Recursive Call


print(fibonacci(15))

The recursive flow of execution of the fibonacci (fib for short) function for n = 4 is as follows:

   fib(4) = fib(3)
             |  |-> fib(3) = fib(2)
             |                |  |-> fib(2) = fib(1)
1.           |                |                |  |-> fib(1) = 1
3.           |                |               (+)
             |                |                |
             |                |               fib(0)
2.           |                |                   |-> fib(0) = 0
5.           |               (+)    
             |                |    
             |               fib(1)
4.           |                   |-> fib(1) = 1      
             |
9.          (+)
             |
            fib(2)
                |-> fib(2) = fib(1)
6.                            |  |-> fib(1) = 1
8.                           (+)
                              |
                             fib(0)
7.                               |-> fib(0) = 0