[python] Else clause on Python while statement

I've noticed the following code is legal in Python. My question is why? Is there a specific reason?

n = 5
while n != 0:
    print n
    n -= 1
else:
    print "what the..."

The better use of 'while: else:' construction in Python should be if no loop is executed in 'while' then the 'else' statement is executed. The way it works today doesn't make sense because you can use the code below with the same results...

n = 5
while n != 0:
    print n
    n -= 1
print "what the..."

In reply to Is there a specific reason?, here is one interesting application: breaking out of multiple levels of looping.

Here is how it works: the outer loop has a break at the end, so it would only be executed once. However, if the inner loop completes (finds no divisor), then it reaches the else statement and the outer break is never reached. This way, a break in the inner loop will break out of both loops, rather than just one.

for k in [2, 3, 5, 7, 11, 13, 17, 25]:
    for m in range(2, 10):
        if k == m:
            continue
        print 'trying %s %% %s' % (k, m)
        if k % m == 0:
            print 'found a divisor: %d %% %d; breaking out of loop' % (k, m)
            break
    else:
        continue
    print 'breaking another level of loop'
    break
else:
    print 'no divisor could be found!'

For both while and for loops, the else statement is executed at the end, unless break was used.

In most cases there are better ways to do this (wrapping it into a function or raising an exception), but this works!

The else clause is executed if you exit a block normally, by hitting the loop condition or falling off the bottom of a try block. It is not executed if you break or return out of a block, or raise an exception. It works for not only while and for loops, but also try blocks.

You typically find it in places where normally you would exit a loop early, and running off the end of the loop is an unexpected/unusual occasion. For example, if you're looping through a list looking for a value:

for value in values:
    if value == 5:
        print "Found it!"
        break
else:
    print "Nowhere to be found. :-("

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

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

My answer will focus on WHEN we can use while/for-else.

At the first glance, it seems there is no different when using

while CONDITION:
    EXPRESSIONS
print 'ELSE'
print 'The next statement'

and

while CONDITION:
    EXPRESSIONS
else:
    print 'ELSE'
print 'The next statement'

Because the print 'ELSE' statement seems always executed in both cases (both when the while loop finished or not run).

Then, it's only different when the statement print 'ELSE' will not be executed. It's when there is a breakinside the code block under while

In [17]: i = 0

In [18]: while i < 5:
    print i
    if i == 2:
        break
    i = i +1
else:
    print 'ELSE'
print 'The next statement'
   ....:
0
1
2
The next statement

If differ to:

In [19]: i = 0

In [20]: while i < 5:
    print i
    if i == 2:
        break
    i = i +1
print 'ELSE'
print 'The next statement'
   ....:
0
1
2
ELSE
The next statement

return is not in this category, because it does the same effect for two above cases.

exception raise also does not cause difference, because when it raises, where the next code will be executed is in exception handler (except block), the code in else clause or right after the while clause will not be executed.

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.

The else-clause is executed when the while-condition evaluates to false.

From the documentation:

The while statement is used for repeated execution as long as an expression is true:

while_stmt ::=  "while" expression ":" suite
                ["else" ":" suite]

This repeatedly tests the expression and, if it is true, executes the first suite; if the expression is false (which may be the first time it is tested) the suite of the else clause, if present, is executed and the loop terminates.

A break statement executed in the first suite terminates the loop without executing the else clause’s suite. A continue statement executed in the first suite skips the rest of the suite and goes back to testing the expression.

The else clause is only executed when your while condition becomes false. If you break out of the loop, or if an exception is raised, it won't be executed.

One way to think about it is as an if/else construct with respect to the condition:

if condition:
    handle_true()
else:
    handle_false()

is analogous to the looping construct:

while condition:
    handle_true()
else:
    # condition is false now, handle and go on with the rest of the program
    handle_false()

An example might be along the lines of:

while value < threshold:
    if not process_acceptable_value(value):
        # something went wrong, exit the loop; don't pass go, don't collect 200
        break
    value = update(value)
else:
    # value >= threshold; pass go, collect 200
    handle_threshold_reached()

The else: statement is executed when and only when the while loop no longer meets its condition (in your example, when n != 0 is false).

So the output would be this:

5
4
3
2
1
what the...

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.