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Why does one part of conditional statements in Python returns None while others are ok?

I need to write a code that prints out valid or invalid for an input of letters and numbers for a license plate. The conditions are: first two characters must be letters, characters have to be between 2 and 6, and if numbers are used, 0 should not be the first, nor should a number appear before a letter.
I put the code below on Thonny and cannot understand why len(l) == 4 part of the conditional statement for def valid_order() returns None and does not execute the next line of the code whereas others work fine. My code should return "Valid" for CSAA50, but it returns invalid. Why?
Also, is there an elegant way to write def valid_order()?
def main():
plate = input("Plate: ")
if is_valid(plate):
print("Valid")
else:
print("Invalid")
def is_valid(s):
if s[0:2].isalpha() and 6 >= len(s) >= 2 and s.isalnum() and valid_order(s):
return True
else:
return False
def valid_order(c):
n = []
l = list(c[2:len(c)])
for i in c:
if i.isdigit():
n += i
if n and n[0] == "0":
return False
if len(l) == 2:
if l[0].isdigit() and l[1].isalpha():
return False
if len(l) == 3:
if l[0].isdigit():
if l[1].isalpha() or l[2].isalpha():
return False
else:
if l[1].isdigit() and l[2].isalpha():
return False
if len(l) == 4:
if l[0].isdigit():
if l[1].isalpha() or l[2].isalpha() or l[3].isalpha():
return False
else:
if l[1].isdigit():
if l[2].isalpha() or l[3].isalpha():
return False
else:
if l[2].isdigit() and l[3].isalpha():
return False
else:
return True
main()

Python Credit Card Validation

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()

Python Palindrome

So my task is to see and check a positive integer if its a palindrome. I've done everything correctly but need help on the final piece. And that the task of generating a new a palindrome from the one given given by the user. Am i on the right track with the while loop or should i use something else? So the result is if you put 192 it would give back Generating a palindrome....
483
867
1635
6996
"""Checks if the given, positive number, is in fact a palindrome"""
def palindrome(N):
x = list(str(N))
if (x[:] == x[::-1]):
return True
else: return False
"""Reverses the given positive integer"""
def reverse_int(N):
r = str(N)
x = r[::-1]
return int(x)
def palindrome_generator():
recieve = int(input("Enter a positive integer. "))
if (palindrome(recieve) == True):
print(recieve, " is a palindrome!")
else:
print("Generating a palindrome...")
while palindrome(recieve) == False:
reverse_int(recieve) + recieve

If I understand your task correctly, the following should do the trick:
def reverse(num):
return num[::-1]
def is_pal(num):
return num == reverse(num)
inp = input("Enter a positive number:")
if is_pal(inp):
print("{} is a palindrome".format(inp))
else:
print("Generating...")
while not is_pal(inp):
inp = str(int(inp) + int(reverse(inp)))
print(inp)
The variable inp is always a string and only converted to int for the arithmetic.

I've been using this solution for many years now to check for palindromes of numbers and text strings.
def is_palindrome(s):
s = ''.join(e for e in str(s).replace(' ','').lower() if e.isalnum())
_len = len(s)
if _len % 2 == 0:
if s[:int(_len/2)] == s[int(_len/2):][::-1]:
return True
else:
if s[int(_len/2+1):][::-1] == s[:int(_len/2)]:
return True
return False

This one is using Complement bitwise and Logical AND and OR operators
_input = 'Abba' # _input = 1221
def isPalindrome(_):
in_str = str(_).casefold() # Convert number to string + case insensitive
for _ in range(int(len(in_str) / 2)): # Loop from 0 till halfway
if in_str[_] != in_str[~_]:
return False
return True
print(_input, isPalindrome(_input) and ' is palindrome' or ' is not palindrome')
Abba is palindrome

Trying to find the next prime number

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

checking whether representation of number in a given base is valid

i have written this code which checks whether a number is correctly represented in a given base. for all the invalid cases it gives false but for true ones it says string index out of range.
def check(n,a,i=0):
if int(n[i])>=a :
return False
else:
return check(n,a,i+1)
n = str(input('enter no:'))
a =int(input('enter base:'))
print(check(n,a,i=0))

The pythonic way:
def is_base_x(num_string, base):
for single_char in num_string:
if int(single_char) >= int(base):
return False
return True

As #ooga pointed out, you need to check when i is larger than the length of your number, you can make it like:
def check(n,a,i=0):
if len(n) <= i:
return True
if int(n[i])>=a :
return False
else:
return check(n,a,i+1)
n = str(input('enter no:'))
a = int(input('enter base:'))
print(check(n,a,i=0))

It would be better if it could check bases above 10. Something like this:
import string
def check(num, base, i = 0):
if i >= len(num):
return True
if not num[i].isdigit():
val = string.ascii_lowercase.find(num[i].lower())
if val == -1 or val + 10 >= base:
return False
elif int(num[i]) >= base:
return False
return check(num, base, i + 1)
while True:
num = raw_input('Enter number: ')
if len(num) == 0: break # null string breaks
base = int(raw_input('Enter base: '))
print(check(num, base))