If .. else in python

Question

I wrote a program in Python to generate prime numbers here is the program

def genPrimes(n):
    primes = [2]  # primes generated so far
    last = 3  # last number tried
    while last <= n:        
        for p in primes:
            if last % p == 0 and math.sqrt(p) <= last:
                break
        else:
            primes.append(last)
        last += 2
    return primes

http://codepad.org/d33tsQyT

This program is producing a right answer. If you see the indentation for else: statement it is wrongly placed. if i try to place the else statement in if block interpreter is showing memory error. Can anyone tell why this is happening.

Thanks in advance Maries

Answer 1

The else is actually attached to the for loop, and executes if the program doesn't break out of the loop. In your case, it executes if none of the primes divide into the number, so the number is prime and gets appended to the list.

See also the documentation.

Answer 2

It's not placed incorrectly, python is assuming you're using a for-else loop.

From the docs:

When used with a loop, the else clause has more in common with the else clause of a try statement than it does that of if statements: a try statement’s else clause runs when no exception occurs, and a loop’s else clause runs when no break occurs.