Else clause on Python while statement

Question

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..." 

Answer 1

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() 

Answer 2

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. :-("