What Does the Percentage Sign Mean in Python

The % does two things, depending on its arguments. In this case, it acts as the modulo operator, meaning when its arguments are numbers, it divides the first by the second and returns the remainder. 34 % 10 == 4 since 34 divided by 10 is three, with a remainder of four.

If the first argument is a string, it formats it using the second argument. This is a bit involved, so I will refer to the documentation, but just as an example:

>>> "foo %d bar" % 5
'foo 5 bar'

However, the string formatting behavior is supplemented as of Python 3.1 in favor of the string.format() mechanism:

The formatting operations described here exhibit a variety of quirks that lead to a number of common errors (such as failing to display tuples and dictionaries correctly). Using the newer str.format() interface helps avoid these errors, and also provides a generally more powerful, flexible and extensible approach to formatting text.

And thankfully, almost all of the new features are also available from python 2.6 onwards.

What does the percentage sign mean?

It's an operator in Python that can mean several things depending on the context. A lot of what follows was already mentioned (or hinted at) in the other answers but I thought it could be helpful to provide a more extensive summary.

% for Numbers: Modulo operation / Remainder / Rest

The percentage sign is an operator in Python. It's described as:

x % y       remainder of x / y

So it gives you the remainder/rest that remains if you "floor divide" x by y. Generally (at least in Python) given a number x and a divisor y:

x == y * (x // y) + (x % y)

For example if you divide 5 by 2:

>>> 5 // 2
2
>>> 5 % 2
1

>>> 2 * (5 // 2) + (5 % 2)
5

In general you use the modulo operation to test if a number divides evenly by another number, that's because multiples of a number modulo that number returns 0:

>>> 15 % 5  # 15 is 3 * 5
0

>>> 81 % 9  # 81 is 9 * 9
0

That's how it's used in your example, it cannot be a prime if it's a multiple of another number (except for itself and one), that's what this does:

if n % x == 0:
    break

If you feel that n % x == 0 isn't very descriptive you could put it in another function with a more descriptive name:

def is_multiple(number, divisor):
    return number % divisor == 0

...

if is_multiple(n, x):
    break

Instead of is_multiple it could also be named evenly_divides or something similar. That's what is tested here.

Similar to that it's often used to determine if a number is "odd" or "even":

def is_odd(number):
    return number % 2 == 1

def is_even(number):
    return number % 2 == 0

And in some cases it's also used for array/list indexing when wrap-around (cycling) behavior is wanted, then you just modulo the "index" by the "length of the array" to achieve that:

>>> l = [0, 1, 2]
>>> length = len(l)
>>> for index in range(10):
...     print(l[index % length])
0
1
2
0
1
2
0
1
2
0

Note that there is also a function for this operator in the standard library operator.mod (and the alias operator.__mod__):

>>> import operator
>>> operator.mod(5, 2)  # equivalent to 5 % 2
1

But there is also the augmented assignment %= which assigns the result back to the variable:

>>> a = 5
>>> a %= 2  # identical to: a = a % 2
>>> a
1