Alternative use of % in Python:

Question

I'm a newbie programmer learning Python. My understanding of the use of the % operator is that it returns the remainder of a division operation. However, I was looking at a piece of code to calculate whether a number in a list is prime, and I ran across a use of this operator that I didn't understand.

def is_prime_number(x):
if x >= 2:
for y in range(2,x):
if not ( x % y ):
return False
else:
return False
return True

In the fourth line, the % operator seems to be returning a boolean result rather than an integer. For the code to work, I think it would have to mean "if x is divisible by y," so x % y would mean "x is not divisible by y."
I set up a test to confirm this.

if 4 % 2:
print("% means 'is divisible by'")
else:
print("% means 'is not divisible by'")
#output: % means 'is not divisible by'

I can't seem to find an explanation of this usage in any searches about the Python operators. Any insight as to why this works as it does would be much appreciated.

Answer 1

The modulus operator (%) does return the remainder of a division.

Just remember that 0 is interpreted as False and any other number as True.

When "x" is divisible by "y" the remainder of their division is 0 (False).

print(bool(4 % 2)) # False

On the other hand, when "x" is not divisible by "y" the remainder of their division is any positive number (True).

print(bool(3 % 2)) # True

Answer 2

if not x%y:

is the exact same as

if x%y == 0:

because if x is divisible by y, it would be 0, the boolean equivalent of false. so,”if 4%2” is false. “not” keyword reverses that.