Credit card validator in Python

Question

I have began taking the CS50 course on EDX the past couple of days, and one of the tasks to accomplish was to write a credit card validator in C. I wrote it in C, and then I thought that I could go about the same thing in Python, and wrote the same program in Python. For those of you unfamiliar with the problem, here is the description.

Because I took the program and converted from C to Python, it is probably not going to be written in the most Pythonic way. I tried to add some Pythonic ways of doing things, but there are probably many more things I could have done. If you find anything that could be done in a better, faster, or more Pythonic way, please let me know. Thanks.

def main():
 # cc_number = int(input("Enter a valid credit card number: "))
 cc_number = 12345678912356789
 if not checksum(cc_number):
 print("INVALID")
 else:
 print(check_provider(cc_number))
def check_provider(number):
 if len(str(number)) < 13 or len(str(number)) > 16:
 return 'INVALID'
 if ((list(str(number)))[:2]) == ['3', '4'] or ((list(str(number)))[:2]) == ['3', '7']:
 return 'AMEX'
 elif ((list(str(number)))[:2]) == ['5', '1'] or ((list(str(number)))[:2]) == ['5', '2'] \
 or ((list(str(number)))[:2]) == ['5', '3'] or ((list(str(number)))[:2]) == ['5', '4'] \
 or ((list(str(number)))[:2]) == ['5', '5']:
 return 'MASTERCARD'
 elif ((list(str(number)))[0]) == '4':
 return 'VISA'
 else:
 return 'INVALID'
def checksum(number):
 list_number = list(str(number))
 odd_indices = []
 even_indices = []
 last_index = len(list_number) - 1
 rev_list = list(reversed(list_number))
 for i in rev_list[1::2]:
 odd_indices.append(i)
 for i in rev_list[::2]:
 even_indices.append(i)
 sum_odd = sum(split_to_digits(''.join(int_to_str(mul(odd_indices, 2)))))
 sum_even = sum(split_to_digits(''.join(int_to_str(even_indices))))
 s = sum_odd + sum_even
 print(s)
 if s % 10 == 0:
 return True
 return False
def mul(list, x):
 return [(int(n) * 2) for n in list]
def split_to_digits(n):
 return [int(i) for i in str(n)]
def int_to_str(n):
 return [str(x) for x in n]
def str_to_int(n):
 return [int(x) for x in n]
main()

Answer 1

General notes:

• use if __name__ == '__main__': to avoid the main() function to be executed when the module is imported
• I'd pass around the credit card number as a string instead of converting it to string in every single validation step
• add docstrings to each of the defined functions

Regarding check_provider() function:

• you can check the length to be in range in one go:

  if not(13 <= len(str(number)) <= 16):
  

• I would improve the way you distinguish between cards by introducing a mapping between brands and regular expressions (like it was done in pycard module). Then, match each of the expressions one by one until you find a match or a brand was not found:

  import re
  BRANDS = {
   'Visa': re.compile(r'^4\d{12}(\d{3})?$'),
   'Mastercard': re.compile(r'''
   ^(5[1-5]\d{4}|677189)\d{10}$| # Traditional 5-series + RU support
   ^(222[1-9]|2[3-6]\d{2}|27[0-1]\d|2720)\d{12}$ # 2016 2-series
   ''', re.VERBOSE),
   'American Express': re.compile(r'^3[47]\d{13}$'),
   'Discover': re.compile(r'^(6011|65\d{2})\d{12}$'),
  }
  def check_provider(number):
   """Checks a credit card number and returns a matching brand name, or INVALID if no brand matched."""
   for brand, regexp in BRANDS.items():
   if regexp.match(number):
   return brand
   return 'INVALID'
  

Regarding implementing the Luhn algorithm, check the pycard's implementation - it is quite clean and understandable.