Recursion
It is legal for a function to
call itself (one of the most magical things a program can do).

def countdown(n):
if n <= 0:
print ("Time!")
else:
print(n)
countdown(n-1)
countdown(3)
Flow
What
happens if we call this function like this?
countdown executions begins with n=3 ...
since n is greater than 0 it outputs 3 ..
then it calls itself ...
countdown executions begins with n=2 ...
since n is greater than 0 it outputs 2 ..
then it calls itself ...
countdown executions begins with n=1 ...
since n is greater than 0 it outputs 1 ..
then it calls itself ...
countdown executions begins with n=0 ...
since n is not greater than 0 it outputs Time! ..
then returns ...
the countdown that got n=1 returns
the countdown that got n=2 returns
the countdown that got n=3 returns
After that we are back in the __main__
Repeat
As another example, we can
print a string n times.

def repeat(s, n):
if n <=0 :
return
print(s)
repeat(s, n-1)
repeat('abc', 3)