Repeating statements with loops

Loops are used to repetitively execute a sequence of statements while changing a variable from iteration to iteration. This variable is called the index variable. It is successively assigned to the elements of a list, (refer to Chapter 9, Iterating) :

L = [1, 2, 10]
for s in L:
    print(s * 2) # output: 2 4 20

The part to be repeated in the for loop has to be properly indented:

for elt in my_list:
    do_something
    something_else
print("loop finished") # outside the for block

Repeating a task

One typical use of a for loop is to repeat a certain task a fixed number of times:

n = 30
for iteration in range(n):
    do_something # this gets executed n times

Break and else

The for statement has two important keywords: break and else. break quits the for loop even if the list we are iterating is not exhausted:

for x in x_values:
   if x > threshold:
     break
   print(x)

The finalizing else checks whether the for loop was broken with the break keyword. If it was not broken, the block following the else keyword is executed:

for x in x_values:
    if x > threshold:
       break
else:
    print("all the x are below the threshold")