Credit card validator using Luhn Algorithm

I’m looking for feedback or tips to maybe make my code more readable or faster or just general tips to help get myself started with app making with Python.

import sys  # check for valid input if len(sys.argv) == 2:     try:         int(sys.argv[1])         if(len(str(sys.argv[1])) == 16):             pass         else:             print("Not 16 digits!")             sys.exit()     except ValueError:         print('Not an integer!')         sys.exit() else:     print('Not enough or too many command line arguments! \n Proper use \"python Check.py <credit card number here> \" ')     sys.exit()  def main():     # put the digits into a list     number = convertToList(sys.argv[1])     sum = cardCheck(number)     if (sum%10 == 0):         print('Valid Card!')     else:         print('Invalid Card!')  #converts initial passed int variable to list def convertToList(num):     numStr = str(num)     numList = []     for digit in numStr:         numList.append(int(digit))     return (numList)  def cardCheck(digitList, count = 0):     sum = 0     #if digit is every second digit multiply by 2     if(count%2 == 0 & count < 15):         digitList[count] = (digitList[count] * 2)         #if is 2 digit number after multiplication         if(digitList[count] >= 10):             digitList[count] = addDigits(digitList[count])             cardCheck(digitList, count + 1)         else:             cardCheck(digitList, count + 1)     #progresses program     elif(count < 15):         cardCheck(digitList, count + 1)     else:         return 0     for digits in digitList:         sum += int(digits)     return sum  #resolve 2 digit number conflict by adding the digits of the number and returning it def addDigits(num):     list = str(num)     sum = 0     for digits in list:         sum += int(digits)     return sum  if __name__ == '__main__':         main()