无法弄清楚为什么我收到UnboundLocalError

Question

I am trying to get the code to "except" a string for an int. - "length". When I put except ValueError, I get back "Please enter a number", but more error is added. I also added to except UnboundLocalError, but that does not seem to work. Please let me know what I'm doing wrong! Here is my code:

import random
import string


def RPG():
    try:
        RPG = ""
        count = 0
        length = int(
            input("How many characters would you like in your password? "))
    except (ValueError, UnboundLocalError):
        print("Please enter a number.")
    while count != length:
        upper = [random.choice(string.ascii_uppercase)]
        lower = [random.choice(string.ascii_lowercase)]
        num = [random.choice(string.digits)]
        symbol = [random.choice(string.punctuation)]
        everything = upper + lower + num + symbol
        RPG += random.choice(everything)
        count += 1
        continue
    if count == length:
        print(RPG)
# could also use string.printable for digits, letters, punct., and whitespace.


RPG()

Here is what I got back from using this code and typing a string instead of an integer into length:

How many characters would you like in your password? j
Please enter a number.
Traceback (most recent call last):
  File "c:\Users\jkelly\Desktop\python\code.py", line 28, in <module>
    pwd()
  File "c:\Users\jkelly\Desktop\python\code.py", line 14, in pwd
    while count != length:
UnboundLocalError: local variable 'length' referenced before assignment

I only expect the "Please enter a number", not the rest of the error, any help would be greatly appreciated. Thank you for your time!

Answer 1

If you cause an error, the rest of the program will still be executed. You need to repeat the input, until you get the correct input.

import random
import string


def RPG():
    while True:
        try:
            RPG = ""
            count = 0
            length = int(
                input("How many characters would you like in your password? "))
            break
        except (ValueError, UnboundLocalError):
            print("Please enter a number.")
    while count != length:
        upper = [random.choice(string.ascii_uppercase)]
        lower = [random.choice(string.ascii_lowercase)]
        num = [random.choice(string.digits)]
        symbol = [random.choice(string.punctuation)]
        everything = upper + lower + num + symbol
        RPG += random.choice(everything)
        count += 1
        continue
    if count == length:
        print(RPG)
# could also use string.printable for digits, letters, punct., and whitespace.


RPG()

Answer 2

The problem with the original code is that while count != length always gets executed, no matter the try-except part. This can be avoided by only proceeding to the while loop if an ValueError or UnboundLocalError was not raised. By initializing c=1 before the try-except and changing it to 0 only in the try part, the program only proceeds to the while loop if exception didn't happen.

import random
import string


def RPG():
    c=0
    try:
        RPG = ""
        count = 0
        length = int(
            input("How many characters would you like in your password? "))
    except (ValueError, UnboundLocalError):
        print("Please enter a number.")
        c=1
    if c==0:
        while count != length:
            upper = [random.choice(string.ascii_uppercase)]
            lower = [random.choice(string.ascii_lowercase)]
            num = [random.choice(string.digits)]
            symbol = [random.choice(string.punctuation)]
            everything = upper + lower + num + symbol
            RPG += random.choice(everything)
            count += 1
            continue
        if count == length:
            print(RPG)
# could also use string.printable for digits, letters, punct., and whitespace.


RPG()