#You were using a logic that applies to any number, use modulo % for this

# 1st example..
a = 1
while a < 1000:
    divisors = [i for i in range(2, a + 1) if a % i == 0]
    if len(divisors) == 1:
        print(a)
    a += 1

# 2nd example..(most comprehensible)
for a in range(1, 1001):
    for i in range(2, a + 1):
        if a % i == 0 and i != a:
            break
    else:
        print(a)


# 3nd example..
for a in range(1, 1001):
    if len([i for i in range(2, a + 1) if a % i == 0]) == 1:
        print(a)

# 4th but not advisable for a total newbie..
maxnum = int(input("Up to what number check primeness?: "))
print('\n'.join(str(a) for a in [n for n in range(1, maxnum + 1) if len([i for i in range(2, maxnum + 1) if n % i == 0]) == 1]))