Question

The task is to get a user to input a password then, using recursion, make sure it has no vowels in it. If it does then let the user re-enter the password. This is what i have so far:

def passwordCheck(pwd):
    """checks if pwd has any vowels in it."""#doc string
    vowels = 'aeiou'#specifies the characters that aren't allowed
    if pwd == '':
        return 0
    elif pwd == None:
        return None#Shouldn't be necessary but just in case
    elif pwd[0] not in vowels:#checks that the 1st(0th) character is not a vowel
        return passwordCheck(pwd[1:])#gets rid of the 1st(0th) character and starts again
    elif pwd[0] in vowels:#checks if the 1st(0th) character is a vowel
        return 1#if it is, stops the function calls and returns a value

password = str(input('Please enter a password with no vowels in it: '))#asks user to input their new password
x = passwordCheck(password)#checks the password is valid, i.e. no vowels

while x == 1:#when the password entered contains a vowel
    print('\nSorry, that is not a valid password.\nYour password cannot contain any vowels.')#tells the user why their password is invalid
    password = str(input('\nPlease enter a different password: '))#gives the user a chance to re-enter their password
    x = passwordCheck(password)#checks to make sure the new password is valid

print('\nCongratulations, you have entered a valid password!')#tells the user if their desired password is valid
print('\nYou are now able to log on to the system with these credentials.')#could've been included on the previous line but looks neater here

I know this is probably not the most pythonic way of doing it but it works for me in most cases. I’d love to hear a better way but ideally someone can help in the same style. I don’t want to just copy someones code without understanding it.

The question i have is dealing with the case where the user enters no password at all. The first if statement:

if pwd == '':
    return 0

I thought it just dealt with the case when the string had been fully recursed through, i.e. no vowels, but after a minutes inspection it’s obvious this applies to no password as well.
I had also tried using:

if pwd == None:
    return something

Now i’m thinking the problem could be because i said:

password = str(input('######'))

but i’ve fiddled with that as well and still can’t can’t seem to make that work either! I’ve tried google and searching stackoverflow but no luck so if anyone has any ideas/solution they think might be helpful I’d be very grateful to hear them. Thank you very much.

My main question is:

How can i differentiate between a string that’s empty because it’s been recursed through and the user inputting nothing?

Solved.

ended up using

def passwordValid(pwd):
if len(pwd)>0 and passwordCheck(pwd)==0:
    return pwd
else: return 'Fail'

password = str(input('Please enter a password with no vowels in it: '))#asks user to input their new password
y = passwordValid(password)#checks the password is valid, i.e. no vowels

while y == 'Fail':#when the password entered contains a vowel
    print('\nSorry, that is not a valid password.\nYour password cannot contain any vowels or be empty.')#tells the user why their password is invalid
    password = str(input('\nPlease enter a different password: '))#gives the user a chance to re-enter their password
    y = passwordValid(password)#checks to make sure the new password is valid


print('\nCongratulations, you have entered a valid password!')#tells the user if their desired password is valid
print('\nYou are now able to log on to the system with these credentials.')#could've been included on the previous line but looks neater here

Thank you Wayne Werner for fixing the title and the main question.

Answer 1

This problem can be broken down into (at least) three distinct subproblems:

• check whether a string contains vowels
• check whether a string is a valid password (length > X and has vowels)
• get a password from the user

Your code should reflect this structure. You could therefore use the following function layout:

def has_vowels(string):
  if not string:  # check for empty string
    return False  # empty strings never have vowels
  # TODO we have a non-empty string at this point and can use recursion

def is_valid_password(string):
  return len(string) > 0 and not has_vowels(string)

def request_password():
  while True:   # use an endless loop here, we don't won't to repeat
                # the "input" statement. We could also change this to
                # something like `for i in range(3)` to allow only a limited
                # number of tries.
    passwd = input('Please enter a password with no vowels in it: ')
    # TODO check if input is valid, if yes, return, if no, print an error