How to protect my python code when a user inputs a string instead of an integer?

Question

I would like to protect the python calculator I have created from crashing when a user inputs a string instead of an integer.

I have tried doing so with an else statement printing "Invalid Input" (or something else I cant remember) when ever a user inputs a string instead of numbers.

I would also like to know if there is a way to let a user do another operation instead of having to restart the application.

If any importing is required (if you can) please list if it is compatible with cx_Freeze.

Source code:

def add (x, y):
    return(x + y)

def subtract(x, y):
    return(x - y)

def multiply(x, y):
    return(x * y)

def divide(x, y):
    return(x / y)

print("Select operation.")
print("1.Add")
print("2.Subtract")
print("3.Multiply")
print("4.Divide")

choice = input("Enter choice(1/2/3/4):")

num1 = int(input("Enter first number: "))
num2 = int(input("Enter second number: "))

if choice == '1':
   print(num1,"+",num2,"=", add(num1,num2))

elif choice == '2':
   print(num1,"-",num2,"=", subtract(num1,num2))

elif choice == '3':
   print(num1,"*",num2,"=", multiply(num1,num2))

elif choice == '4':
   print(num1,"/",num2,"=", divide(num1,num2))
else:
   print("Invalid input")

Dmitry.Samborskyi · Accepted Answer · 2015-07-06 07:49:45Z

7

you can use something like this for input

while True:
    try:
        num1 = int(input("Enter first number: "))
    except ValueError:
        continue
    else:
        break

answered Jul 6, 2015 at 7:49

Dmitry.Samborskyi

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3 Comments

Alok Mysore Over a year ago

I was writing a similar answer. Why not put that code in a function so it can be reused? :)

Dmitry.Samborskyi Over a year ago

@AlokShankar can be. Just write an example and I think author should think is there a need for func or not :)

Alok Mysore Over a year ago

s/he is using the same functionality twice to get two inputs which is why I made the comment, but like you said, It is up to the author.

Martin Evans · Accepted Answer · 2015-07-06 08:49:52Z

2

Take a look at the changes I have made to your code as follows:

def add (x, y):
    return(x + y)

def subtract(x, y):
    return(x - y)

def multiply(x, y):
    return(x * y)

def divide(x, y):
    return(x / y)

def input_number(prompt):
    while True:
        try:
            return int(input(prompt))
        except ValueError:
            print("That was not a number")

# Keep going around the loop until the user chooses 5 to quit

while True:
    print
    print("Select operation.")
    print("1.Add")
    print("2.Subtract")
    print("3.Multiply")
    print("4.Divide")
    print("5.Quit")

    choice = input("Enter choice(1/2/3/4/5):")

    # Do they want to quit?

    if choice == 5:
        break

    num1 = input_number("Enter first number: ")
    num2 = input_number("Enter second number: ")

    if choice == 1:
        print(num1,"+",num2,"=", add(num1,num2))

    elif choice == 2:
        print(num1,"-",num2,"=", subtract(num1,num2))

    elif choice == 3:
        print(num1,"*",num2,"=", multiply(num1,num2))

    elif choice == 4:
        print(num1,"/",num2,"=", divide(num1,num2))

    else:
        print("%s - Invalid input" % choice)

In order to ask for more input, you needed to wrap your prompts in a loop. You would then need to add an option to the user to allow them to quit.

Also, you can move the prompting for numbers to a function. This would keep asking for number if the user typed in a character.

edited Jul 6, 2015 at 8:49

answered Jul 6, 2015 at 8:01

Martin Evans

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1 Comment

SiHa Over a year ago

Bare except is generally a bad idea. Whilst I accept that in this case it's probably OK, it is not a good example to suggest.

martianwars · Accepted Answer · 2015-07-06 07:53:37Z

1

This code snippet should help :-

def is_string(test_data):
    if type(test_data) is str:
        return True
    else:
        return False

answered Jul 6, 2015 at 7:53

martianwars

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