For - else

else Clause

Executes after the loop completes normally. Ie., the loop did not encounter a breakstatement.

Common use case

Run a loop to search for an item. If the item is found, we break out of the loop using the break statement.

2 scenarios in which the loop may end:

1. when the item is found and break is encountered.

2. The loop ends without encountering a break statement.

We may want to know which one of these is the reason for a loop’s completion. One method is to set a flag and then check it once the loop ends. Another is to use the else clause.

Bluprint

for item in container:
    if search_something(item):
        # Found it!
        process(item)
        break
else:
    # Didn't find anything..
    not_found_in_container()

Example

Consider this example: It finds factors for numbers between 2 to 10.:

for n in range(2, 10):
    for x in range(2, n):
        if n % x == 0:
            print(n, 'equals', x, '*', n/x)
            break

We can add an additional elseblock which catches the numbers which have no factors and are therefore prime numbers:

for n in range(2, 10):
    for x in range(2, n):
        if n % x == 0:
            print( n, 'equals', x, '*', n/x)
            break
    else:
        # loop fell through without finding a factor
        print(n, 'is a prime number')