How to multiply every other list item by 2 in python -

i'm making program validates credit card, multiplying every other number in card number 2; after i'll add digits multiplied 2 ones not multiplied 2. of double digit numbers added sum of digits, 14 becomes 1+4. have photo below explains all. i'm making python program of steps. i've done code below it, have no idea next? please help, , appreciated. code have returns error anyway.

class validator():     def __init__(self):         count = 1         self.card_li = []         while count <= 16:             try:                 self.card = int(input("enter number "+str(count)+" of card number: "))                 self.card_li.append(self.card)                 #print(self.card_li)                 if len(str(self.card)) > 1:                     print("only enter 1 number!")                     count -= 1             except valueerror:                count -= 1            count += 1         self.validate()      def validate(self):         self.card_li.reverse()         #print(self.card_li)         count = 16         while count >= 16:             self.card_li[count] = self.card_li[count] * 2             count += 2       validator() 

to perform sum:

>>> s = '4417123456789113' >>> sum(int(c) c in ''.join(str(int(x)*(2-i%2)) i, x in enumerate(s))) 70 

how works

the code consists of 2 parts. first part creates string every other number doubled:

>>> ''.join(str(int(x)*(2-i%2)) i, x in enumerate(s)) '8427226410614818123' 

for each character x in string s, converts character integer, int(x) , multiplies either 1 or 2 depending on whether index, i, or odd: (2-i%2). resulting product converted string: str(int(x)*(2-i%2)). strings joined together.

the second part sums each digit in string:

>>> sum(int(c) c in '8427226410614818123') 70