Write a program primes.py that reads a positive integer from standard input, and determines whether or not the number is prime.

Sure, here is a simple Python program that checks if a number is prime:

def is_prime(n):
    if n <= 1:
        return False
    elif n <= 3:
        return True
    elif n % 2 == 0 or n % 3 == 0:
        return False
    i = 5
    while i * i <= n:
        if n % i == 0 or n % (i + 2) == 0:
            return False
        i += 6
    return True

n = int(input("Enter a positive integer: "))
if is_prime(n):
    print(n, "is a prime number")
else:
    print(n, "is not a prime number")

This program works by checking divisibility of n starting from 2 up to the square root of n. If n is divisible by any of these numbers, then it is not a prime number. If n is not divisible by any of these numbers, then it is a prime number.