Else clause on Python while statement

The else clause is only executed when the while-condition becomes false.

Here are some examples:

Example 1: Initially the condition is false, so else-clause is executed.

i = 99999999

while i < 5:
    print(i)
    i += 1
else:
    print('this')

OUTPUT:

this

Example 2: The while-condition i < 5 never became false because i == 3 breaks the loop, so else-clause was not executed.

i = 0

while i < 5:
    print(i)
    if i == 3:
        break
    i += 1
else:
    print('this')

OUTPUT:

0
1
2
3

Example 3: The while-condition i < 5 became false when i was 5, so else-clause was executed.

i = 0

while i < 5:
    print(i)
    i += 1
else:
    print('this')

OUTPUT:

0
1
2
3
4
this

I know this is old question but...

As Raymond Hettinger said, it should be called while/no_break instead of while/else.
I find it easy to understeand if you look at this snippet.

n = 5
while n > 0:
    print n
    n -= 1
    if n == 2:
        break
if n == 0:
    print n

Now instead of checking condition after while loop we can swap it with else and get rid of that check.

n = 5
while n > 0:
    print n
    n -= 1
    if n == 2:
        break
else:  # read it as "no_break"
    print n

I always read it as while/no_break to understand the code and that syntax makes much more sense to me.

Else is executed if while loop did not break.

I kinda like to think of it with a 'runner' metaphor.

The "else" is like crossing the finish line, irrelevant of whether you started at the beginning or end of the track. "else" is only not executed if you break somewhere in between.

runner_at = 0 # or 10 makes no difference, if unlucky_sector is not 0-10
unlucky_sector = 6
while runner_at < 10:
    print("Runner at: ", runner_at)
    if runner_at == unlucky_sector:
        print("Runner fell and broke his foot. Will not reach finish.")
        break
    runner_at += 1
else:
    print("Runner has finished the race!") # Not executed if runner broke his foot.

Main use cases is using this breaking out of nested loops or if you want to run some statements only if loop didn't break somewhere (think of breaking being an unusual situation).

For example, the following is a mechanism on how to break out of an inner loop without using variables or try/catch:

for i in [1,2,3]:
    for j in ['a', 'unlucky', 'c']:
        print(i, j)
        if j == 'unlucky':
            break
    else: 
        continue  # Only executed if inner loop didn't break.
    break         # This is only reached if inner loop 'breaked' out since continue didn't run. 

print("Finished")
# 1 a
# 1 b
# Finished

Suppose you've to search an element x in a single linked list

    def search(self, x):
        position = 1
        p =self.start
        while p is not None:
            if p.info == x:
                print(x, " is at position ", position)
                return True
            position += 1
            p = p.link
        else:
            print(x, "not found in list") 
            return False

So if while conditions fails else will execute, hope it helps!

As far as I know the main reason for adding else to loops in any language is in cases when the iterator is not on in your control. Imagine the iterator is on a server and you just give it a signal to fetch the next 100 records of data. You want the loop to go on as long as the length of the data received is 100. If it is less, you need it to go one more times and then end it. There are many other situations where you have no control over the last iteration. Having the option to add an else in these cases makes everything much easier.