Filters non-prime numbers in Python when working with large data sets.

Primes.py

def next_down(number):
    number -=1
    for i in range(3): #there will be a max of three iterations here (6,5,4)
        if number %2 == 0 or number %5 == 0:
            number -=1
        else:
            break
    return number

def sum_digits(numstr):
    sum = 0
    for digit in numstr:
        sum += int(digit)
    return sum

def last_two_digits(numstr):
    return int(numstr[-2:])

def last_three_digits(numstr):
    return int(numstr[-3:])

def seven_test(numstr):
    last_digit = numstr[-1]
    double = int(last_digit)*2
    is_divis = int(numstr[:-1])-double
    rem = is_divis % 7
    return (rem == 0) or (is_divis == 0)

def eleven_test(numstr):
    part_one = int(numstr[1])
    part_two = int(numstr[0])
    i = 2
    while i < len(numstr):
        if i % 2 ==0:
            part_two += int(numstr[i])
        else:
            part_one += int(numstr[i])
        i+=1
    diff = part_one - part_two
    if (diff == 0) or (diff % 11 == 0):
        return True

def thirteen_test(numstr):
    last_digit = int(numstr[-1])
    all_but_last = int(numstr[:-1])
    nine_x = last_digit*9
    return (all_but_last - nine_x) % 13 == 0

def worth_testing(num):
    as_str = str(num)
    # Divis by 5, 10
    if as_str.endswith('5'):
        return False

    # Divis by 3 
    if sum_digits(as_str) % 3 == 0:
        return False

    # Divis by 7
    if seven_test(as_str):
        return False

    # Divis by 9
    if sum_digits(as_str) % 9 == 0:
        return False

    # Divis by 11
    if eleven_test(as_str):
        return False

    # Divis by 13
    if thirteen_test(as_str):
        return False
    return True

def is_prime(number):
    # Filters out numbers divisibly by
    # 1-13 
    sqrt = math.sqrt(number)
    if (sqrt % 1.0 == 0.0):
        return False
    
    if not worth_testing(number):
        return False
    
    # If a number has a whole square root, 
    # it is not prime

    iter_factor = math.ceil(sqrt)

    # We only need to test for numbers less than
    # the square root that are prime
    test_numbers = primes_less_than(iter_factor)

    for num in test_numbers:
        if (number % num ==0):
            return False

    return True