"""

Checks a given ISIN for validity

reference to: https://en.wikipedia.org/wiki/International_Securities_Identification_Number

reference to: http://code.activestate.com/recipes/498277-isin-validator/

:param string_isin: ISIN to be checked

:return: None in case the string does not meet format requirements, True in case the ISIN is valid,

false in case not

"""

# check the ISIN to be exactly 12 alphanumeric characters

if len(string_isin) != 12 or not string_isin.isalnum():

return None

# check the ISIN to start with two letters

if not string_isin[:2].isalpha():

return None

# check the ISIN to end with a digit

if not string_isin[-1].isnumeric():

return None

list_isin_parts = []

# Convert alpha characters to digits

for each_char in string_isin:

if each_char.isdigit():

list_isin_parts.append(int(each_char))

else:

# increase alphabetical index by 9

list_isin_parts.append(string.ascii_uppercase.index(each_char.upper()) + 9 + 1)

# last digit serves as check digit

check_digit = list_isin_parts[-1]

list_isin_parts = list_isin_parts[:-1]

# convert each int into a string

list_isin_parts = ''.join([str(i) for i in list_isin_parts])

# separate even and odd indices

isin_even = list_isin_parts[::2]

isin_odd = list_isin_parts[1::2]

# If len(isin2) is odd, multiply evens by 2, else multiply odds by 2

if len(list_isin_parts) % 2 > 0:

isin_even = ''.join([str(int(i)*2) for i in list(isin_even)])

else:

isin_odd = ''.join([str(int(i)*2) for i in list(isin_odd)])

even_sum = sum([int(i) for i in list(isin_even)])

odd_sum = sum([int(i) for i in list(isin_odd)])

# calculate check digit

mod = (even_sum + odd_sum) % 10

check_digit_calc = (10 - mod) % 10

if check_digit == check_digit_calc:

return True

else: