for_-_else.rst

for/else

Loops are an integral part of any language. Likewise for loops are an important part of Python. However there are a few things which most beginners do not know about them. We will discuss a few of them one-by-one.

Let's first start off with what we know. We know that we can use for loops like this:

fruits = ['apple', 'banana', 'mango']
for fruit in fruits:
    print(fruit.capitalize())

# Output: Apple
#         Banana
#         Mango

That is the very basic structure of a for loop. Now let's move on to some of the lesser known features of for loops in Python.

else Clause

for loops also have an else clause which most of us are unfamiliar with. The else clause executes after the loop completes normally. This means that the loop did not encounter a break statement. They are really useful once you understand where to use them. I, myself, came to know about them a lot later.

The common construct is to run a loop and search for an item. If the item is found, we break out of the loop using the break statement. There are two scenarios in which the loop may end. The first one is when the item is found and break is encountered. The second scenario is that the loop ends without encountering a break statement. Now 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.

This is the basic structure of a for/else loop:

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

Consider this simple example which I took from the official documentation:

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

It finds factors for numbers between 2 to 10. Now for the fun part. We can add an additional else block 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')