Question

For a bit of python practice, I decided to work on a calculator tutorial. It was very basic, so I decided to give it exception handling in the event a user enters in garbage. While proper use of the program still works, punching in crap still causes it to crash, and entering
Here is my code:

loop = 1

choice = 0

while loop == 1:
    #print out the options you have
    print "Welcome to calculator.py"

    print "your options are:"

    print " "
    print "1) Addition"
    print "2) Subtraction"

    print "3) Multiplication"

    print "4) Division"
    print "5) Quit calculator.py"
    print " "

    choice = input("choose your option: ")
    try:
        if choice == 1:
            add1 = input("add this: ")
            add2= input("to this: ")
            print add1, "+", add2, "=", add1+ add2
        elif choice == 2:
            sub1 = input("Subtract this ")
            sub2 = input("from this")
            print sub1, "-", sub2, "=", sub1 - sub2
        elif choice == 3:
            mul1 = input("Multiply this: ")
            mul2 = input("with this: ")
            print mul1, "x", mul2, "=", mul1 * mul2
        elif choice == 4:
            div1 = input("Divide this: ")
            div2 = input("by this: ")
            if div2 == 0:
                print "Error! Cannot divide by zero!  You'll destroy the universe! ;)"
            else:

                print div1, "/", div2, "=", div1 * div2
        elif choice == 5:
            loop = 0
        else:
            print "%d is not valid input. Please enter 1, 2 ,3 ,4 or 5." % choice

    except ValueError:
        print "%r is not valid input.  Please enter 1, 2, 3, 4 or 5." % choice
    print "Thank you for using calculator.py!"

Now while I found an useable answer here: Error Handling Variables in a calculator program, Error handling numbers are fine

I was wondering why my code didn’t work. Does python want the exception handling in a function? That’s the vibe I’m getting from it.

Answer 1

In Python 2 (which is what you are using) input evaluates as Python code whatever the user enters. Because of this input can raise many different exceptions, but rarely a ValueError.

Better would be to accept your input with the raw_input which returns a string, and then cast to the expected type. If the input is invalid it will then raise a ValueError:

>>> x = int(raw_input("enter something: "))
enter something: sdjf
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: 'sdjf'

Note: In Python 3 input assumes the semantics of Python 2’s raw_input and raw_input goes away.