A recursion s the process of repeating items in a self-similar way. In computer programming, a recursive function is a function that calls itself.

There are several classic recursive functions, like a function to calculate the factoral¹⁾ of a given number :

def factorial(n):
   if n == 1:
      return 1
   else:
      return n * factorial(n-1)

To calculate the factorial of -per example- the integer 5 (written as 5!) you feed the integer 5 a an parameter to the factorial function above:

print factorial(5)

Recursions can also apper in graphics, like on this ad for Droste cacao (see Droste_effect): A wouman holding a packet of cacao with the image of a wouman holding a packet of cacao with the image of a wouman holding a packet of cacao….

You can use python's turtle module to generate beautiful patterns like trees out of very few lines of code:

import turtle as t
 
def y(length=100):
    """ paints a branch of a tree with 2 smaller branches, like an Y"""
    if length < 10:
           return       # escape the function
    t.fd(length)        # paint the thik branch of the tree
    t.left(30)          # rotate left for smaller "fork" branch
    y(length * 0.6)      # create a smaller branch with 2/3 the lenght of the parent branch
    t.right(60)         # rotoate right for smaller "fork" branch
    y(length * 0.6)      # create second smaller branch
    t.left(30)          # rotate back to original heading
    t.fd(-length)       # move back to original position
    return              # leave the function, continue with calling program
 
# calling the function the first time
y(200) # create a tree, the first branch has a length of 200 pixel