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How can I make it so that the program loops until the conditions are met?

I was trying to make it so that my program will keep asking the user to input a certain value and if the user doesn't it keeps asking until they do.
I tried to use "while" instead of "if" but I know I'm probably missing something, somewhere.
def terrain(surface):
surface = raw_input("What surface will you be driving on? ")
if surface == "ice":
u = raw_input("what is the velocity of the car in meters per second? ")
u = int(u)
if u < 0:
u = raw_input("Velocity must be greater than 0")
return
if u == 0:
u = raw_input("Velocty must be a number greater than zero")
return
a = raw_input("How quickly is the vehicle decelerating? ")
a = int(a)
if a > 0:
print ("Deceleration cannot be a positive integer")
return
else:
s1 = u**2
s2 = 2*.08*9.8
s = s1/s2
print "This is how far the vehicle will travel on ice: "
print ("The vehicle will travel %i meters before coming to a complete stop" % (s))
terrain("ice")

The problem is you are using return after checking the condition which causes the function to return None you have to use break instead of return with while loop instead of if to achieve this. A better way to validate and get data is below
class ValidationError(Exception):
pass
def validate_and_get_data_count(x):
if int(x) < 0:
raise ValidationError("Cannot be less than 0")
return int(x)
def validate_and_get_data_name(x):
if len(x) < 8:
raise ValidationError("Length of name cannot be less than 8 chars")
elif len(x) > 10:
raise ValidationError("Length of name cannot be greater than 10 chars")
return x
validators = {
"count": validate_and_get_data_count,
"name": validate_and_get_data_name
}
data = {}
params = [
("count","Please enter a count: "),
("name","Please enter a name: "),
]
for param in params:
while True:
x = input(param[1])
try:
data[param[0]] = validators[param[0]](x)
break
except ValidationError as e:
print(e)
print(data)
What the above code does is for every param in params list it runs a while loop checking for every validation condition defined in its validator if valid it breaks the while loop and proceeds to next param and repeats the same process again

Change from base Shadocks to Base 10 in Python

I have a little problem who block me, I've a work where I must to convert a number to Shadocks (base 4 it seems), and I must to make a decrypter.
So I made the first part, but my code won't work on the second.
Here it's :
def Base10toShadocks(n):
q = n
r = 0
Base4=[]
Shads=["GA","BU","ZO","MEU"]
if q == 0:
Base4.append(0)
else:
while q > 0:
q = n//4
r = n%4
n = q
Base4.append(r)
Base4.reverse()
VocShad = [Shads[i] for i in Base4]
print(VocShad)
def ShadockstoBase10(n):
l=len(n)
Erc_finale=[]
for i in range(l):
Sh=(n[i])
i=i+1
if Sh =="a":
Erc_finale.append(0)
elif Sh =="b":
Erc_finale.append(1)
elif Sh =="o":
Erc_finale.append(2)
elif Sh =="e":
Erc_finale.append(3)
print(Erc_finale)
F=str(Erc_finale)
print(F)
F=F.replace("[","")
F=F.replace("]","")
F=F.replace(",","")
F=F.replace(" ","")
L2=len(F)
F=int(F)
print(L2)
print(F)
r=0
while f < 4 or F ==4:
d=(F%4)-1
F=F//4
print(d)
r=r+d*(4**i)
print(r)
inp = 0
inp2 = 0
print("Tapez \"1\" pour choisir de traduire votre nombre en shadock, ou \"2\" pour inversement")
inp = int(input())
if inp == 1:
print("Quel est le nombre ?")
inp2 = int(input())
if inp2 != None:
Base10toShadocks(inp2)
elif inp == 2:
print("Quel est le nombre ?")
inp2 = str(input())
if inp2 != None:
ShadockstoBase10(inp2)
It blocks at the F=int(F), I don't understand why.
Thanks for your help.

First, some errors in your code:
for i in range(l):
Sh=(n[i])
i=i+1 #### Won't work. range() will override
##### Where do "a","b","o","e" come from
##### Shouldn't it be "G","B","Z","M" ("GA","BU","ZO","MEU")?
if Sh =="a":
Erc_finale.append(0)
elif Sh =="b":
Erc_finale.append(1)
elif Sh =="o":
Erc_finale.append(2)
elif Sh =="e":
Erc_finale.append(3)
print(Erc_finale)
F=str(Erc_finale) ### Not how you join an array into a string
Here's a corrected way:
def ShadockstoBase10(n):
n = n.upper(); # Convert string to upper case
l = len(n)
Erc_finale = "" # Using a string instead of an array to avoid conversion later
i = 0
while i < l: # while loop so we can modify i in the loop
Sh = n[i:i+2] # Get next 2 chars
i += 2 # Skip 2nd char
if Sh == "GA":
Erc_finale += "0"
elif Sh == "BU":
Erc_finale += "1"
elif Sh == "ZO":
Erc_finale += "2"
elif Sh =="ME" and "U" == n[i]:
Erc_finale += "3"
i += 1; # MEU is 3 chars
else:
break; # bad char
return int(Erc_finale, 4) # Let Python do the heavy work
Like everything in Python, there are other ways to do this. I just tried to keep my code similar to yours.

checking if the input is int >=0 using if statement

IDnum = input("\nprompt: ")
if int(IDnum) >= 0 :
if int(IDnum) in T.keys() :
print("ID number(s) that {} will contact is(are) {}.".format(int(IDnum),T[int(IDnum)]))
else :
print("Entered ID number {} does not exist.".format(int(IDnum)))
else:
break
It's actually a while loop, receiving ID numbers and checking whether the numbers are in the file.
I'd like to make it discern whether the input is an integer >= 0 and if it's anything else, (eg. space,enter,characters,float,etc) break the loop.
How can I do this using if statements?
I have tried
if IDnum == '' or IDnum == ' ' or int(IDnum) < 0 :
but as you know, it cannot cover all the other cases.

T = {1: 1, 2: 2}
while True:
IDnum = input("\nprompt: ")
try:
num = int(IDnum)
if num < 0:
raise ValueError('Negative Integers not allowed')
except ValueError: # parsing a non-integer will result in exception
print("{} is not a valid positive integer.".format(IDnum))
break
if num in T:
print("ID number(s) that {} will contact is(are) {}.".format(num,T[num]))
else:
print("Entered ID number {} does not exist.".format(num))
Thanks to #adirio and #moses-koledoye for the suggested improvements.

Do the check with a try-except statement.
def is_pos_int(IDnum):
''' Check if string contains non-negative integer '''
try:
number = int(IDnum)
except ValueError:
return False
if number >= 0:
return True
else:
return False
For example
is_pos_int('1 ') # notice the space
Out[12]: True
is_pos_int('-1')
Out[13]: False
is_pos_int('1.0')
Out[15]: False
is_pos_int('word')
Out[16]: False
Then:
while True:
if not is_pos_int(IDnum):
break
else:
val = int(IDnum)
if val in T.keys() :
print("ID number(s) that {} will contact is(are) {}.".format(val, T[val]))
else :
print("Entered ID number {} does not exist.".format(val))

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()

Can you use break to validate input in python?

Like using this to validate that an input is only alpha-numeric:
while True:
str = input('')
if str.isalnum():
break
else:
print("Please include only alpha-numeric characters.\n")
This code has worked for all instances that I have tested it in, but is this bad practice?

That's fine. Here is a note, however: you can find out if the while loop exited with a break or without one by using else:
x = 0
while x < 4:
x += 1
else:
print("no break")
# prints no break
If you break, however:
x = 0
while x < 4:
x += 1
if x == 2:
break
else:
print("no break")
# Does not print

you can abstract it further
def verified_input(prompt='',test_condition=lambda x:1,err_message="Please Enter Valid Input"):
while True:
result = input(prompt)
if test_condition(result):
return result
print( err_message )
def verified_alnum(prompt,err_message="Please enter Only alpha numerics"):
return verified_input(prompt,lambda x:x.isalnum(),err_message)
result = verified_alnum("Enter Password:","A password must be only letters and numbers")
this allows you to create any number of test conditions quickly and relatively verbosely