Fibonacci series program in Python

In the Fibonacci series, each number is the sum of the two numbers before it. It starts 0, 1, and continues 1, 2, 3, 5, 8, 13, ...

The program

n = int(input("How many terms? "))

if n <= 0:
    print("Please enter a number greater than 0.")
else:
    a, b = 0, 1
    terms = []
    for _ in range(n):
        terms.append(a)
        a, b = b, a + b
    print(*terms)

Typed in this example: 10. Press Run to type your own.

How many terms? 10
0 1 1 2 3 5 8 13 21 34

How it works

• a holds the current number and b holds the next one.
• a, b = b, a + b updates both at the same time. Python calculates the right side first (using the old a and b), and then stores both results. With two separate lines (a = b, then b = a + b), the second line would use the new a and give wrong numbers.
• _ is a common name for a loop variable that you do not use.
• print(*terms) prints the items of the list separated by spaces, without the brackets.

Numbers below a limit

When you want every Fibonacci number below a value (instead of a fixed count), a while loop fits better:

limit = 100

a, b = 0, 1
while a < limit:
    print(a, end=" ")
    a, b = b, a + b
print()

0 1 1 2 3 5 8 13 21 34 55 89 

end=" " makes print() put a space after each number instead of starting a new line. The last print() ends the line.