How do you check whether a number is divisible by another number?

I need to test whether each number from 1 to 1000 is a multiple of 3 or a multiple of 5.

I tried this code in Python 2.x:

n = 0s = 0while (n < 1001):    x = n/3    if isinstance(x, (int, long)):        print 'Multiple of 3!'        s = s + n    if False:        y = n/5        if isinstance(y, (int, long)):            s = s + n    print 'Number: '    print n    print 'Sum:'    print s    n = n + 1

The idea is to try dividing the number and see if the result is an integer. However, I'm not getting the expected result.

How do I test whether the number is an integer?

_{In 2.x, division like this will produce an integer, discarding the remainder; see How can I force division to be floating point? Division keeps rounding down to 0? for details.}

_{In 3.x, the division will produce a floating-point value; the result is not"an integer" even if it is a whole number, so the isinstance check will fail. See Why does integer division yield a float instead of another integer? for details.}

_{If you need the remainder from integer division rather than just testing for divisibility, see Find the division remainder of a number.}