Question

I am a Python beginner. I find that the else in for–else and while–else is completely unnecessary. Because for and while will finally run to else, and we can use the usual lines instead.

For example:

for i in range(1, 5):
    print i
else:
    print 'over'

and

for i in range(1, 5):
    print i
print 'over'

are the same.

So why does Python have else in for–else and while–else?

Answer 1

You are wrong about the semantics of for/else. The else clause runs only if the loop completed, for example, if a break statement wasn’t encountered.

The typical for/else loop looks like this:

for x in seq:
    if cond(x):
        break
else:
    print "Didn't find an x I liked!"

Think of the “else” as pairing with all of the “if’s” in the loop body. Your samples are the same, but with “break” statements in the mix, they are not.

A longer description of the same idea: http://nedbatchelder.com/blog/201110/forelse.html