How to Implement a Fibonacci Sequence in Python

If you're invited for a coding interview, one of the classic questions you might be asked is how would you code a Fibonacci series? For obvious reasons, you'll first need to know what's a Fibonacci series before you can implement the solution. Then you'll need to decide which language you will use to implement the solution. Let's first with a simple definition of the Fibonacci series and later we will focus on its implementation using Python. 

What is a Fibonacci Sequence?

The Fibonacci Sequence (F_n) is the series of numbers such that each number is calculated by adding the sum of the two numbers before it, with the first two terms being 0 and 1.

0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233...

From a mathematical standpoint, the Fibonacci Sequence is defined by a recurrence relation and is defined using the following rule. 

                                                    $F\left(n\right) = F(n-1) + F(n-2)$

where:

• F(n) is the term number “n”
• F(n-1) is the previous term (n-1)
• F(n-2) is the term before that (n-2)

So let's implement the Fibonacci Sequence using the recursive formula in Python. Before you dive in you'll need to understand the following programming concepts in Python.

• if statements
• for loops
• functions
• recursion

def fib_recursive(n):
    if n <= 1:
        return n
    else:
        return (fib_recursive(n-1) + fib_recursive(n-2))

no_terms = 14

#check n<0
if no_terms < 0:
    print("Invalid input, please enter a positive integer.")
else:
    print("The first 14 terms of Fibionacci Sequence are as follows." )
    for i in range(no_terms):
       print(fib_recursive(i)) 

Output 

The first 14 terms of Fibionacci Sequence are as follows.
0
1
1
2
3
5
8
13
21
34
55
89
144
233

Keep the momentum going in preparation for your interview and check out my article on the four pillars of Object-Oriented Programming here.