I'm a beginner Python learner and I'm currently working on Luhn Algorithm to check credit card validation. I wrote most of the code, but I'm stuck with 2 errors I get 1st one is num is referenced before assignment. 2nd one I'm getting is object of type '_io.TextIOWrapper' has no len(). Further help/ guidance will be greatly appreciated.
These are the steps for Luhn Algorithm (Mod10 Check)
Double every second digit from right to left. If this “doubling” results in a two-digit number, add the two-digit
number to get a single digit.
Now add all single digit numbers from step 1.
Add all digits in the odd places from right to left in the credit card number.
Sum the results from steps 2 & 3.
If the result from step 4 is divisible by 10, the card number is valid; otherwise, it is invalid.
Here's what my output is supposed to be
Card Number Valid / Invalid
--------------------------------------
3710293 Invalid
5190990281925290 Invalid
3716820019271998 Valid
37168200192719989 Invalid
8102966371298364 Invalid
6823119834248189 Valid
And here is the code.
def checkSecondDigits(num):
length = len(num)
sum = 0
for i in range(length-2,-1,-2):
number = eval(num[i])
number = number * 2
if number > 9:
strNumber = str(number)
number = eval(strNumber[0]) + eval(strNumber[1])
sum += number
return sum
def odd_digits(num):
length = len(num)
sumOdd = 0
for i in range(length-1,-1,-2):
num += eval(num[i])
return sumOdd
def c_length(num):
length = len(num)
if num >= 13 and num <= 16:
if num [0] == "4" or num [0] == "5" or num [0] == "6" or (num [0] == "3" and num [1] == "7"):
return True
else:
return False
def main():
filename = input("What is the name of your input file? ")
infile= open(filename,"r")
cc = (infile.readline().strip())
print(format("Card Number", "20s"), ("Valid / Invalid"))
print("------------------------------------")
while cc!= "EXIT":
even = checkSecondDigits(num)
odd = odd_digits(num)
c_len = c_length(num)
tot = even + odd
if c_len == True and tot % 10 == 0:
print(format(cc, "20s"), format("Valid", "20s"))
else:
print(format(cc, "20s"), format("Invalid", "20s"))
num = (infile.readline().strip())
main()
You just forgot to initialize num
def main():
filename = input("What is the name of your input file? ")
infile= open(filename,"r")
# initialize num here
num = cc = (infile.readline().strip())
print(format("Card Number", "20s"), ("Valid / Invalid"))
print("------------------------------------")
while cc!= "EXIT":
even = checkSecondDigits(num)
odd = odd_digits(num)
c_len = c_length(num)
tot = even + odd
if c_len == True and tot % 10 == 0:
print(format(cc, "20s"), format("Valid", "20s"))
else:
print(format(cc, "20s"), format("Invalid", "20s"))
num = cc = (infile.readline().strip())
First, maybe you should remove the extra characters:
def format_card(card_num):
"""
Formats card numbers to remove any spaces, unnecessary characters, etc
Input: Card number, integer or string
Output: Correctly formatted card number, string
"""
import re
card_num = str(card_num)
# Regex to remove any nondigit characters
return re.sub(r"\D", "", card_num)
After check if credit card is valid using the Luhn algorithm:
def validate_card(formated_card_num):
"""
Input: Card number, integer or string
Output: Valid?, boolean
"""
double = 0
total = 0
digits = str(card_num)
for i in range(len(digits) - 1, -1, -1):
for c in str((double + 1) * int(digits[i])):
total += int(c)
double = (double + 1) % 2
return (total % 10) == 0
This is a very simpler version of code it is based on lunh's algorithm
def validator(n):
validatelist=[]
for i in n:
validatelist.append(int(i))
for i in range(0,len(n),2):
validatelist[i] = validatelist[i]*2
if validatelist[i] >= 10:
validatelist[i] = validatelist[i]//10 + validatelist[i]%10
if sum(validatelist)%10 == 0:
print('This a valid credit card')
else:
print('This is not valid credit card')
def cardnumber():
result=''
while True:
try:
result = input('Please enter the 16 digit credit card number : ')
if not (len(result) == 16) or not type(int(result) == int) :
raise Exception
except Exception:
print('That is not a proper credit card number. \nMake sure you are entering digits not characters and all the 16 digits.')
continue
else:
break
return result
def goagain():
return input('Do you want to check again? (Yes/No) : ').lower()[0] == 'y'
def main():
while True:
result = cardnumber()
validator(result)
if not goagain():
break
if __name__ == '__main__':
main()
Old thread but the answer concerns me... and the real issue wasn't identified.
Actually, the error is that you have used the identifier (num) for the parameter when defining checkSecondDigits as the identifier/name of the argument when calling the function in the mainline. The function should be called in main() by
even = checkSecondDigits(cc) so the value in cc (which is the argument) is passed into num (as the parameter) for use within the function.
The same rookie error is made with odd_digits and cc_length.
This question (and the initially suggested answer) demonstrates a fundamental mis-understanding of passing arguments to parameters...
The suggested 'declaring' of num just hides this error/misunderstanding and also obfuscates the local and global scopes of num (which should only be local) and cc (which is global) so whilst the suggestion works in this case, it works for the wrong reason and is poor style and bad programming.
Further,
num should not appear anywhere in main() as it should be local to (only appear inside of) the functions called...
The last line in this code should be the same as the first, but the last line incorrectly assigns the data to num instead of cc
cc = (infile.readline().strip())
print(format("Card Number", "20s"), ("Valid / Invalid"))
print("------------------------------------")
while cc!= "EXIT":
even = checkSecondDigits(num)
odd = odd_digits(num)
c_len = c_length(num)
tot = even + odd
if c_len == True and tot % 10 == 0:
print(format(cc, "20s"), format("Valid", "20s"))
else:
print(format(cc, "20s"), format("Invalid", "20s"))
num = (infile.readline().strip())
you can use my code for card validation it is 100% dynamic because of the card structure is stored in CSV file, so it is easy to update here is the code on GitHub profile, python file link, code explanation file link and CSV for datafile link
python code:
# -*- coding: utf-8 -*-
"""
Created on Tue Sep 10 20:55:30 2019
#author: Preyash2047#gmail.com
"""
import csv
import numpy as np
#csv file imported and storf in reader
reader = csv.DictReader(open("card_data.csv"))
#input card number
card_number = input("Enter the card No: ")
#global variable declaration
min_digits=0
max_digits=0
card_number_list = list(card_number)
card_number_list_reverse=card_number_list[::-1]
card_number_length=len(card_number_list)
first_digit = int(card_number_list[0])
#global variable for final output
card_provider_list_number = 0
result_found = False
card_number_digits = 0
mit_name=""
#list
start=[]
end=[]
name=[]
c_d=[]
number_length=[]
min_max_digits_list=[]
#append the list from csv
for raw in reader:
start.append(raw['start'])
end.append(raw['end'])
name.append(raw['name'])
c_d.append(raw['c_d'])
number_length.append(raw['number_length'])
#initialize the value of min_digits & max_digits
def min_max_digits():
global min_digits
global max_digits
for i in range(len(start)):
available_length=number_length[i].split(',')
for j in range(len(available_length)):
min_max_digits_list.append(available_length[j])
min_max_digits_array = np.array(min_max_digits_list)
np.unique(min_max_digits_array)
min_digits=int(min(min_max_digits_array))
max_digits=int(max(min_max_digits_array))
#list to int
def list_to_int(noofdigits):
str1 = ""
return int(str1.join(noofdigits))
#card validation
def iin_identifier():
first_six_digit = list_to_int(card_number_list[0:6])
for i in range(len(start)):
if(first_six_digit >= int(start[i]) and first_six_digit <= int(end[i])):
available_length=number_length[i].split(',')
for j in range(len(available_length)):
if(card_number_length == int(available_length[j])):
global card_provider_list_number
card_provider_list_number = i
global card_number_digits
card_number_digits = available_length[j]
global result_found
result_found = True
#Major Industry Identifier (MII) identification
def mit_identifier():
global first_digit
global mit_name
switcher = {
1: "Airlines",
2: "Airlines",
3: "Travel and Entertainment",
4: "Banking and Financial Services",
5: "Banking and Financial Services",
6: "Merchandising and Banking",
7: "Petroleum",
8: "Health care, Telecommunications",
9: "National Assignment"
}
mit_name=switcher.get(first_digit, "MIT Identifier Not Found")
#Luhn Algorithm or modulo-10 Algorithm
def luhn_algorithm():
for i in range(card_number_length):
if(i%2!=0 and i!=0):
card_number_list_reverse[i]=int(card_number_list_reverse[i])*2
#print(str(i)+" "+ str(card_number_list_reverse[i]))
if(len(str(card_number_list_reverse[i]))==2):
even_number_2=list(str(card_number_list_reverse[i]))
card_number_list_reverse[i] = int(even_number_2[0])+int(even_number_2[1])
#print("\tsubsum "+str(i)+" "+str(card_number_list_reverse[i]))
else:
card_number_list_reverse[i]=int(card_number_list_reverse[i])
division_int = int(sum(card_number_list_reverse)/10)
division_float=sum(card_number_list_reverse)/10
if(division_int-division_float==0):
return True
#initial level number length validation
def card_number_validation():
min_max_digits()
if(card_number_length>= min_digits and card_number_length <= max_digits and first_digit != 0):
iin_identifier()
mit_identifier()
if(result_found and luhn_algorithm()):
print("\nEntered Details are Correct\n")
print("\nHere are the some details we know about you card")
print("\nNo: "+card_number)
print("\nIssuing Network: "+name[card_provider_list_number])
print("\nType: "+c_d[card_provider_list_number]+" Card")
print("\nCategory of the entity which issued the Card: "+mit_name)
else:
print("\nCard Number is Invalid\nPlease renter the number!\n")
else:
print("\nCard Number is Invalid\n")
#method called to run program
card_number_validation()
n = input("Enter 16-digit Credit Card Number:")
lst = []
for i in range(16):
lst.append(n[i])
# print(lst)
# list1 = n.split()
# print(list1)
def validate_credit_card():
global lst
if len(lst) == 16:
for i in range(0, len(lst)):
lst[i] = int(lst[i])
# print(lst)
last = lst[15]
first = lst[:15]
# print(first)
# print(last)
first = first[::-1]
# print(first)
for i in range(len(first)):
if i % 2 == 0:
first[i] = first[i] * 2
if first[i] > 9:
first[i] -= 9
sum_all = sum(first)
# print(first)
# print(sum_all)
t1 = sum_all % 10
t2 = t1 + last
if t2 % 10 is 0:
print("Valid Credit Card")
else:
print("Invalid Credit Card!")
else:
print("Credit Card number limit Exceeded!!!!")
exit()
if __name__ == "__main__":
validate_credit_card()
Related
im doing a small "mastermind" game for a project, seems to run fine up until my last while loop, i thought i have an end statement but it seems to run on repeat. I've been stuck on this for some time now and i would appreciate any and all help on this, thanks! Here is the code:
import random
def generate_code():
"""Create a random code as a list"""
for i in range(0,4):
i = random.randint(0,5)
code.append(i)
print(code)
def make_guess():
"""Let's the user input a guess"""
while len(guess) < 4:
element = input("your guess, one at the time: " )
if element.isnumeric():
element = int(element)
global amountOfGuesses
if element in range(0,6):
guess.append(element)
amountOfGuesses = amountOfGuesses +1
else:
print("number has to be between 0 and 5")
else:
print("has to be a number between 0 and 5")
def right_position(guess, code):
"""Calculate how many correkt numbers on right position the guess have"""
howManyRight = 0
for i in range(4):
if guess[i] == code[i]:
howManyRight = howManyRight +1
return howManyRight
def wrong_position(guess, code):
"""Calculate how many numbers are corret but wrong position"""
howManyWrongPosition = 0
tempCode = code[:]
for i in guess:
if i in tempCode:
tempCode.remove(i)
howManyWrongPosition = howManyWrongPosition +1
howManyWrongPosition = howManyWrongPosition - right_position(guess, code)
return howManyWrongPosition
code = []
guess = []
wrongPosition = []
rightPosition = []
codeCopy = code.copy()
amountOfGuesses = 0
print("Welcome to Mastermind.\nYou get seven guesses to gues a random 4 digit code with 6 different numbers between 0 and 5.")
generate_code()
while amountOfGuesses <= 7:
make_guess()
print("you have", right_position(guess, code), "right numbers on the right position")
print("you have", wrong_position(guess, code), "numbers on that is right but on the wrong posiotion")
if guess[:] == code[:]:
print("Congratulation you won!!! you used", amountOfGuesses, "guesses.")
From what I understand you want one try to be one input of 4 numbers, so I also fixed that. The reason you're getting an infinite loop is because you haven't broken out of the loop at end. You should also clear the guess array, otherwise the for loop inside the make_guess() will just skip due to the length being 4 (in case the guess was wrong and want to try again).
The fixed code (assuming one try is input of 4 numbers):
import random
def generate_code():
"""Create a random code as a list"""
for i in range(0,4):
i = random.randint(0,5)
code.append(i)
print(code)
def make_guess():
"""Let's the user input a guess"""
global amountOfGuesses
while len(guess) < 4:
element = input("your guess, one at the time: " )
if element.isnumeric():
element = int(element)
if element in range(0,6):
guess.append(element)
else:
print("number has to be between 0 and 5")
else:
print("has to be a number between 0 and 5")
amountOfGuesses = amountOfGuesses +1
def right_position(guess, code):
"""Calculate how many correkt numbers on right position the guess have"""
howManyRight = 0
for i in range(4):
if guess[i] == code[i]:
howManyRight = howManyRight +1
return howManyRight
def wrong_position(guess, code):
"""Calculate how many numbers are corret but wrong position"""
howManyWrongPosition = 0
tempCode = code[:]
for i in guess:
if i in tempCode:
tempCode.remove(i)
howManyWrongPosition = howManyWrongPosition +1
howManyWrongPosition = howManyWrongPosition - right_position(guess, code)
return howManyWrongPosition
code = []
guess = []
wrongPosition = []
rightPosition = []
codeCopy = code.copy()
amountOfGuesses = 0
print("Welcome to Mastermind.\nYou get seven guesses to gues a random 4 digit code with 6 different numbers between 0 and 5.")
generate_code()
while 1:
make_guess()
print("you have", right_position(guess, code), "right numbers on the right position")
print("you have", wrong_position(guess, code), "numbers on that is right but on the wrong posiotion")
if guess == code:
print("Congratulation you won!!! you used", amountOfGuesses, "guesses." if amountOfGuesses > 1 else "guess.")
break
elif amountOfGuesses > 7:
print(f"You have lost by using {amountOfGuesses} tries!")
break
guess = []
from cs50 import get_int, get_string
#Asking User For Credit Card Number
def main():
while True:
Credit_Card = str(get_int("Enter Credit Card Number: "))
if Credit_Card != "":
break
cardCheck(cardSum, Credit_Card)
def cardSum(Credit_Card):
even = 0
odd = 0
CC = len(Credit_Card)
if (CC == 0):
return 0
else:
if (CC % 2 == 0):
last = int(Credit_Card[-1])
even += last
return even + cardSum(Credit_Card[:-1])
else:
last = int(Credit_Card[-1])
last = last * 2
oddDigits = last // 10 + last % 10
odd += oddDigits
return odd + cardSum(Credit_Card[:-1])
#Checking is card valid or not
def cardCheck(cardSum,Credit_Card):
Total = cardSum(Credit_Card)
if (Total % 10 == 0):
if (len(Credit_Card) == 15) and (int(Credit_Card[0:2]) in [37, 34]):
print("AMEX")
elif (len(Credit_Card) == 16) and (int(Credit_Card[:2]) in range(50, 56)):
print("MASTER CARD")
elif (len(Credit_Card) in [13, 16]) and (int(Credit_Card[:1]) == 4):
print("VISA")
else:
print("INVALID")
else:
print("INVALID")
if __name__ == "__main__":
main()
I cannot figure it out Why my Program is not giving correct output with these AMEX Card Number:378282246310005 , 371449635398431 In My C implemented credits these two card are showing AMEX
but in python it showing INVALID as output
It's working Fine For Master,Visa
I believe the problem is your code is using an absolute sense of parity instead of a relative one. That is, we start doubling with the second to last digit on the right and then every other digit going left. But the parity of the indicies of the digits to double depends on whether the original number had an odd or even number of digits. Your code doesn't account for this.
We need to calculate the parity of the card at the start, and pass this parity throught the recursive cardSum() calls, so it doubles the correct digits:
def cardSum(digits, parity):
if not digits:
return 0
digit, digits = int(digits[-1]), digits[:-1]
if len(digits) % 2 == parity:
digit *= 2
digit = digit // 10 + digit % 10
return digit + cardSum(digits, parity)
def cardCheck(cardSum, credit_card):
checksum = cardSum(credit_card, len(credit_card) % 2)
if checksum % 10 == 0:
if len(credit_card) == 15 and int(credit_card[0:2]) in [34, 37]:
print("AMEX")
elif len(credit_card) == 16 and int(credit_card[:2]) in range(50, 56):
print("MASTER CARD")
elif len(credit_card) in [13, 16] and int(credit_card[:1]) == 4:
print("VISA")
else:
print("INVALID")
else:
print("INVALID")
if __name__ == "__main__":
credit_card = input("Enter Credit Card Number: ")
if credit_card != "":
cardCheck(cardSum, credit_card)
My guess is this comes up with AMEX because it is an odd length number -- did you test odd length numbers from any other card issuer?
I want a program so I can enter a big integer and it will do a calculation like this:
62439 = (6 - 2 + 4 - 3 + 9)
Basically it splits them up and the first and third should be added and the second and the fourth should be subtracted. Please follow the loop sort of method. I can't have the elif and if working in the same run of the loop.
num = input("Input any number: ")
total = 0
p = 0
x= 0
q = 0
number = len(num)
if len(num) ==5 :
total = 0
for i in range(0,(number)):
p = p+2
q = q +1
if x == 2 and q == p:
total = total+(int(num[q]))
x=1
p=p-1
elif x == 1 and q == p :
total = total-(int(num[q]))
x=2
p=p-1
print("your number is: ",total)
I expect it to repeat the loop as many times as there are numbers in the integer that is entered e.g (333) executes 3 times
num = input("Enter a number: ")
sum = 0
do_add = True
try:
for digit in num:
if do_add:
sum += int(digit)
else:
sum -= int(digit)
do_add = not do_add
except Exception:
print("Invalid input")
else:
print("The result is", sum)
Why not just perform the calculation:
num = input("Input any number: ")
if len(num) == 5:
total = int(num[0])-int(num[1])+int(num[2])-int(num[3])+int(num[4])
print("your number is: ",total)
Maybe like this:
def main():
digits = input("Enter a number: ")
assert digits.isdigit()
result = sum(int(digit) * [1, -1][index%2] for index, digit in enumerate(digits))
print(f"Result: {result}")
return 0
if __name__ == "__main__":
import sys
sys.exit(main())
Or like this if you wanna be more deliberate - also easier to extend:
def main():
from operator import add, sub
digits = input("Enter a number: ")
assert digits.isdigit()
operations = [add, sub]
result = 0
for index, digit in enumerate(digits):
result = operations[index%len(operations)](result, int(digit))
print(f"Result: {result}")
return 0
if __name__ == "__main__":
import sys
sys.exit(main())
Or just go overkill (also known as "taking advantage of the standard library"):
def main():
from itertools import cycle
from operator import add, sub
from functools import reduce
while True:
digits = input("Enter a number: ")
if digits.isdigit():
break
operation_iter = cycle([sub, add])
def operation(a, b):
return next(operation_iter)(a, b)
result = reduce(operation, map(int, digits))
print(f"Result: {result}")
return 0
if __name__ == "__main__":
import sys
sys.exit(main())
MyFunctions file file -
def factList(p,n1):
counter = 1
while counter <= n1:
if n1 % counter == 0:
p.append(counter)
counter = counter + 1
def isPrime(lst1,nbr):
factList(lst1, nbr)
if len(lst1) == 2:
return True
else:
return False
def nextPrime(nbr1):
cnt1 = 1
while cnt1 == 1:
nbr1 == nbr1 + 1
if isPrime(lst2,nbr1):
cnt1 = 0
Filetester file -
nbr1 = 13
nextPrime(nbr1)
print nbr1
My isPrime function already works I'm tring to use my isPrime function for my nextPrime function, when I run this I get
">>>
13
" (when using 13)
">>> " (When using 14)
I am supposed to get 17 not 13. And if I change it to a composite number in function tester it gets back in a infinite loop. Please only use simple functions (the ones I have used in my code).
This is NOT the right way to do this, but this is the closest adaptation of your code that I could do:
def list_factors_pythonic(number):
"""For a given number, return a list of factors."""
factors = []
for x in range(1, number + 1):
if number % x == 0:
factors.append(x)
return factors
def list_factors(number):
"""Alternate list_factors implementation."""
factors = []
counter = 1
while counter <= number:
if number % counter == 0:
factors.append(counter)
return factors
def is_prime(number):
"""Return true if the number is a prime, else false."""
return len(list_factors(number)) == 2
def next_prime(number):
"""Return the next prime."""
next_number = number + 1
while not is_prime(next_number):
next_number += 1
return next_number
This would be helpful:
def nextPrime(number):
for i in range(2,number):
if number%i == 0:
return False
sqr=i*i
if sqr>number:
break
return True
number = int(input("Enter the num: ")) + 1
while(True):
res=nextPrime(number)
if res:
print("The next number number is: ",number)
break
number += 1
I don't know python but if it's anything like C then you are not assigning anything to your variables, merely testing for equality.
while cnt1 == 1:
nbr1 == nbr1 + 1
if isPrime(lst2,nbr1):
cnt1 == cnt1 + 1
Should become
while cnt1 == 1:
nbr1 = nbr1 + 1 << changed here
if isPrime(lst2,nbr1):
cnt1 = cnt1 + 1 << and here
Well this code help you
n=int(input())
p=n+1
while(p>n):
c=0
for i in range(2,p):
if(p%i==0):
break
else:c+=1
if(c>=p-2):
print(p)
break
p+=1
this code optimized for finding sudden next prime number of a given number.it takes about 6.750761032104492 seconds
def k(x):
return pow(2,x-1,x)==1
n=int(input())+1
while(1):
if k(n)==True:
print(n)
break
n=n+1
I am working on a Hangman game, but I am having trouble replacing the dashes with the guessed letter. The new string just adds on new dashes instead of replacing the dashes with the guessed letter.
I would really appreciate it if anyone could help.
import random
import math
import os
game = 0
points = 4
original = ["++12345","+*2222","*+33333","**444"]
plusortimes = ["+","*"]
numbers = ["1","2","3"]
#FUNCTIONS
def firstPart():
print "Welcome to the Numeric-Hangman game!"
def example():
result = ""
ori = random.choice(original)
for i in range(2,len(ori)):
if i % 2 == 0:
result = result + ori[i] + ori[0]
else:
result = result + ori[i] + ori[1]
return ori
# def actualGame(length):
#TOP LEVEL
firstPart()
play = raw_input("Do you want to play ? Y - yes, N - no: ")
while (play == "Y" and (points >= 2)):
game = game + 1
points = points
print "Playing game #: ",game
print "Your points so far are: ",points
limit = input("Maximum wrong guesses you want to have allowed? ")
length = input("Maximum length you want for the formulas (including symbols) (must be >= 5)? ")
result = "" #TRACE
ori = random.choice(original)
for i in range(2,len(ori)):
if i % 2 == 0:
result = result + ori[i] + ori[0]
else:
result = result + ori[i] + ori[1]
test = eval(result[:-1])
v = random.choice(plusortimes) #start of randomly generated formula
va = random.choice(plusortimes)
formula = ""
while (len(formula) <= (length - 3)):
formula = formula + random.choice(numbers)
formula2 = str(v + va + formula)
kind = ""
for i in range(2,len(formula2)):
if i % 2 == 0:
kind = kind + formula2[i] + formula2[0]
else:
kind = kind + formula2[i] + formula2[1]
formula3 = eval(kind[:-1])
partial_fmla = "------"
print " (JUST TO TRACE, the program invented the formula: )" ,ori
print " (JUST TO TRACE, the program evaluated the formula: )",test
print "The formula you will have to guess has",length,"symbols: ",partial_fmla
print "You can use digits 1 to 3 and symbols + *"
guess = raw_input("Please enter an operation symbol or digit: ")
a = 0
new = ""
while a<limit:
for i in range(len(formula2)):
if (formula2[i] == partial_fmla[i]):
new = new + partial_fmla[i]
elif (formula2[i] == guess):
new[i] = guess
else:
new[i] =new + "-"
a = a+1
print new
guess = raw_input("Please enter an operation symbol or digit: ")
play = raw_input("Do you want to play ? Y - yes, N - no: ")
The following block seems problematic:
elif (formula2[i] == guess):
new[i] = guess
else:
new[i] =new + "-"
Python does not allow modification of characters within strings, as they are immutable (cannot be changed). Try appending the desired character to your new string instead. For example:
elif formula2[i] == guess:
new += guess
else:
new += '-'
Finally, you should put the definition of new inside the loop directly under, as you want to regenerate it after each guess.