Why adding try except block in if statement and the program says variable not defined

Question

I want to write a program that is about tools and has three choices--calculator, random number selector, and calendar. However, when I am trying to use try-except for invalid user input, the variable in the if statement is said as undefined. The codes are the following:

        if user_input == 0:
        try:
            small_option = int(
            input("Which operation do you want to do? \n 1. Adding \n 2. Subtracting \n 3. Multiplying "
                  "\n 4. Dividing"))
            small_list = [0, 1, 2, 3]
            user_option = small_list[small_option]
        except IndexError:
            print("Invalid option")

Then the next variable in the if statement is undefined, the following lines:

                if user_option == 0:
                num1 = float(input("Enter first number: "))
                num2 = float(input("Enter second number: "))

When I ran it, the program says Traceback (most recent call last): File "C:/Users/31459/PycharmProjects/Useful programs/Tools.py", line 41, in <module> if user_option == 0: NameError: name 'user_option' is not defined

import calendar
import random




def adding(para1, para2):
    return para1 + para2



def substract(para1, para2):
    return para1 - para2



def multiplying(para1, para2):
    return para1 * para2



def dividing(para1, para2):
    return para1 / para2


while True:
    try:
        big_option = input("Which tool will you use? \n 0. Calculator \n 1. Calender \n 2. Random number generator")
        big_list = [0, 1, 2]
        big_option_int = int(big_option)
        user_input = big_list[big_option_int]
        if user_input == 0:
            try:
                small_option = int(
                input("Which operation do you want to do? \n 1. Adding \n 2. Subtracting \n 3. Multiplying "
                      "\n 4. Dividing"))
                small_list = [0, 1, 2, 3]
                user_option = small_list[small_option]
            except IndexError:
                print("Invalid option")
                if user_option == 0:
                    num1 = float(input("Enter first number: "))
                    num2 = float(input("Enter second number: "))
                if user_option == 1:
                    print("The sum is", num1, "+", num2, adding(num1, num2))
                elif user_option == 2:
                    print("The subtraction is", num1, "-", num2, substract(num1, num2))
                elif user_option == 3:
                    print("The multiplying is", num1, "*", num2, multiplying(num1, num2))
                else:
                    print("The divide is", num1, "/", num2, dividing(num1, num2))
                    break
            if user_option == 1:
                user_year_str = input("Please input the year you want to get.")
                user_month_str = input("Please input the month you want to get.")
                user_year = int(user_year_str)
                user_month = int(user_month_str)
                calendar.month(user_year, user_month)
            else:
                random_input_1 = input("Please input the 1st number:")
                random_input_2 = input("Please input the 2nd number:")
                random_input_1_int = int(random_input_1)
                random_input_2_int = int(random_input_2)
                random.randint(random_input_1_int, random_input_2_int)
    except IndexError:
        print("Invalid option")

Answer 1

the user_option needs to be defined

are you sure you defined the variable before using the conditional statement?

it needs to have a value before using the conditional statement

Answer 2

It's probably because the try block raises an error and it fails to initialize variable user_option . Hence, it raises an error.

Try adding this as an alternative

user_option = small_option -1

Answer 3

Every object(here: variable) in python has a scope, and in that scope is recognizable. out of its scope it's unrecognizable.

The scope of user_option variable is the try block that in it user_option is defined. if you want to use user_option out of try block you should defined it in bigger scope (means out of try block that it's defined).

Answer 4

You're making option 4 invalid by accessing the list by direct index.

Indices start at 0, so consequently option 4 does not exist, that is an Index Error. 1-3 does not raise an error as the user_option is defined without exception. But your "0" option in small_list will never be used.

Simply initializing the variable user_option before the try/catch will fix the UnboundLocalError . But not the flaw in your logic. You could subract the given option value by one to turn it into a 0-starting index as you have defined the small_list variable

small_list[small_option-1] will retrieve 0 when you write 1 and 3 when you write 4, all of which are defined indices of small_list .

user_option = None        # by defining the variable first, we avoid the UnboundLocalError
try:
    small_option = int(
    input("Which operation do you want to do? \n 1. Adding \n 2. Subtracting \n 3. Multiplying "
            "\n 4. Dividing"))
    small_list = [0, 1, 2, 3]
    user_option = small_list[small_option-1]        # 1. Adding = small_list[0], 2. Subtracting = small_list[1], etc..
except IndexError:
    print("Invalid option")
if user_option == 0:
    num1 = float(input("Enter first number: "))
    num2 = float(input("Enter second number: "))