How to calculate a mod b in Python?

There's the % sign. It's not just for the remainder, it is the modulo operation.

you can also try divmod(x, y) which returns a tuple (x // y, x % y)

>>> 15 % 4
3
>>>

The modulo gives the remainder after integer division.

mod = a % b

This stores the result of a mod b in the variable mod.

And you are right, 15 mod 4 is 3, which is exactly what python returns:

>>> 15 % 4
3

a %= b is also valid.

Why don't you use % ?

print 4 % 2 # 0

A = [3, 1, 2, 4]
for a in A:
    print(a % 2)

output:

1
1
0
0