Ben Klemp Ben Klemp - 5 months ago 29
Python Question

Faulty Random Password Generator (Python 3)

I am beginning to learn Python and started experimenting with an example code block. I edited it a few times, and on the last edit that I did, I added an optional random password generator. Then I decided that it would make more sense to put the password generator into a separate document, so I copied the necessary code and made a new document. After editing it however, I cannot generate an even number of digits in the password.

Pastebin

Copy of Faulty Code (Pastebin)

import math
import random
alpha = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z']
print('Would you like a random password suggestion generator', 'Yes or No')
permissionRandomGenerator = input().lower()
print('How long do you want your password?')
lengthRandomGenerator = int(input())
if permissionRandomGenerator == 'yes':
def randInt():
return math.floor(random.random()*10)
def randChar():
return alpha[math.floor(random.random()*27)]
randPasswordList = []
listInsert = 0
def changeCase(f):
g = round(random.random())
if g == 0:
return f.lower()
elif g == 1:
return f.upper()
while listInsert < lengthRandomGenerator:
randPasswordList.insert(listInsert, randInt())
listInsert = listInsert + 1
if listInsert >= lengthRandomGenerator:
break
randPasswordList.insert(listInsert, randChar())
randPasswordList[listInsert] = changeCase(randPasswordList[listInsert])
listInsert = listInsert + 1
continue
listInsert = 0
printList = 0
if lengthRandomGenerator <= 0:
print('It has to be longer than that')
elif lengthRandomGenerator >= 25:
print('I can\'t generate a password that long')
elif math.isnan(lengthRandomGenerator):
print('error: not valid data type')
else:
while printList < (len(randPasswordList)-1):
printItem = randPasswordList[printList]
print(printItem)
printList = printList + 1
printList = 0
randPasswordList = []
elif permissionRandomGenerator == 'no':
print('Too bad...')
else:
print('You had to answer Yes or No')

Answer

I refactored your program a bit, and got rid of a lot of unnecessary steps and inconsistencies. Here it is in full, then I'll explain each part:

import random
import string
import sys

possible_chars = string.ascii_letters + string.digits + string.punctuation

def nextchar(chars):
    return random.choice(chars)

yes_or_no = input("""
Would you like a random password suggestion generated?
Type Yes to continue: """).lower()

if yes_or_no == 'yes':
    try:
        pwd_len = int(input('How long do you want your password? '))
    except ValueError:
        sys.exit("You need to enter an integer. Please start the program over.")

    if 0 < pwd_len < 26:
        new_pwd = ""
        for _ in range(pwd_len):
            new_pwd += nextchar(possible_chars)
        print("Your new password is:\n" + new_pwd)

    else:
        print("I can only generate passwords between 1 and 25 characters long.")

else:
    print("Well then, why did you run me?")

Python is not just the syntax and builtin functions, it is also the standard library or stdlib. You're going to be working with the stdlib's modules all the time, so when you think you'll be using one, read the docs! You'll learn about the module, what its intended use is, some of its history and changes (such as in which version a certain function was added), and all of the classes, functions, and attributes contained therein. Make sure you read the whole thing (none of them are that long) and try to get at least a basic idea of what each thing does. That way, such as in this case, you'll be able to pick the best function for the job. One thing I like to do in my spare time is just pick a random module and read the docs, just to learn. They're generally fairly well written, and usually pretty inclusive. Get used to Monty Python references, they're everywhere.

import random
import string
import sys

Imports are first, and should almost always be only at the top. I like to put mine in alphabetical order, with the stdlib on top, then a blank line, then 3rd-party modules, including self-written ones next. Put a blank line or two after the imports as well. One thing to remember, that I mentioned in the comments: readability counts. Code is not only meant to be read by machines, but by people as well. Comment when necessary. Be generous with whitespace (also remember that whitespace is syntactically important in Python as well, so it forces you to indent properly) to separate related bits of code, functions, classes, blocks, etc. I highly recommend reading, rereading, and spending time pondering PEP-8, the Python style guide. Its recommendations aren't absolute, but many projects that enforce coding standards rely on it. Try to follow it as much as you can. If a line comes out to 83 characters, don't sweat it, but be aware of what you're doing.

The reason I made such a big deal out of reading the docs is the following few lines:

possible_chars = string.ascii_letters + string.digits + string.punctuation

def nextchar(chars):
    return random.choice(chars)

They get rid of about half of your code. string contains a bunch of predefined constants for working with strings. The three I chose should all be good valid password characters. If you're on a system that won't take punctuation marks, just remove it. Note that possible_chars is a string - like tuples, lists and dicts, strings are iterable, so you don't need to make a separate list of each individual possible character.

Next is the function - it replaces your randInt(), randChar(), and changeCase() functions, along with a bunch of your inline code, which was rather bizarre, to tell you the truth. I liked the method you came up with to decide if a letter was upper- or lower-case, but the rest of it was just way too much effort when you have random.choice() and the string constants from above.

yes_or_no = input("""
Would you like a random password suggestion generated?
Type Yes to continue: """).lower()

You may not have been aware, but you don't need to print() a description string before getting user input() - just pass the string as a single argument to input() and you'll get the same effect. I also used a triple-quoted """ (''' can also be used) string literal that differs from the more common single- ' and double-quoted " string literals in that any newlines or tabs contained within it don't need to be escaped. The take-home for now is that you can write several lines of text, and when you print() it, it will come out as several lines.

    try:
        pwd_len = int(input('How long do you want your password? '))
    except ValueError:
        sys.exit("You need to enter an integer. Please start the program over.")

I used a try/except block for the next part. If the user enters a non-integer up at the input prompt, the int() function will fail with a ValueError. I picked the simplest manner possible of dealing with it: if there's an error, print a message and quit. You can make it so that the program will re-ask for input if an error is raised, but I figured that was beyond the scope of this exercise.

    if 0 < pwd_len < 26:
        new_pwd = ""
        for _ in range(pwd_len):
            new_pwd += nextchar(possible_chars)
        print("Your new password is:\n" + new_pwd)

    else:
        print("I can only generate passwords between 1 and 25 characters long.")

Here is where all the action happens. Using an if/else block, we test the desired length of the password, and if it's between 1 and 25 (an arbitrary upper bound), we generate the password. This is done with a for loop and the range() function (read the docs for exactly how it works). You'll notice that I use a common Python idiom in the for loop: since I don't actually need the number generated by range(), I "throw it away" by using the underscore _ character in place of a variable. Finally, the else statement handles the alternative - either pwd_len is 0 or less, or 26 or greater.

else:
    print("Well then, why did you run me?")

We're at the end of the program! This else is paired with the if yes_or_no == 'yes': statement - the user entered something other than yes at the input prompt.

Hopefully this will help you understand a little bit more about how Python works and how to program efficiently using it. If you feel like you're spending a bit too much time implementing something that you think should be easier, you're probably right. One of Python's many advantages is its "batteries included" philosophy - there's a huge range of things you can do with the stdlib.

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