Question

I am trying to modify the code below to check the input from a user. If the input is a string I want to generate an error see the function choiceTest(). But this doesn’t work and I get the following exception, can you help
graham

Choice your option:k
Traceback (most recent call last):
  File "C:\Users\Graham\bkup-workspaces\workspace\python-1\calculator.py", line 54, in <module>
    choice = menu()
  File "C:\Users\Graham\bkup-workspaces\workspace\python-1\calculator.py", line 18, in menu
    return input ("Choice your option:")    
  File "C:\Users\Graham\Desktop\letsussee\eclipse-java-helios-win32\eclipse\plugins\org.python.pydev_2.5.0.2012040618\PySrc\pydev_sitecustomize\sitecustomize.py", line 210, in input
    return eval(raw_input(prompt))
  File "<string>", line 1, in <module>
NameError: name 'k' is not defined

The code:

# calculator program

# NO CODE IS REALLY RUN HERE, IT IS ONLY TELLING US WHAT WE WILL DO LATER
# Here we will define our functions
# this prints the main menu, and prompts for a choice
def menu():
    #print what 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 " "
        return input ("Choice your option:")    
# this adds two numbers given

def add(a,b):
    print a, "+", b, "=", a + b

# this subtracts two numbers given
def sub(a,b):
    print b, "-", a, "=", b - a

# this multiplies two numbers given
def mul(a,b):
    print a, "*", b, "=", a * b

# this divides two numbers given
def div(a,b):
    print a, "/", b, "=", a / b

# decide if the choice is valid or not 
def choiceTest():


    isinstanceValue = isinstance(choice,int)
    instancevalueout = str(isinstanceValue)
    print "value is" + instancevalueout
    if not instancevalueout is "True":
       print " NOT AN INTEGER " + instancevalueout
       raise SystemExit

# NOW THE PROGRAM REALLY STARTS, AS CODE IS RUN
loop = 1
choice = 0
isinstanceValue = True
instancevalueout = ""

while loop == 1:
    choice = menu()
    choiceTest()
    if choice == 1:
        add(input("Add this: "),input("to this: "))
    elif choice == 2:
        sub(input("Subtract this: "),input("from this: "))
    elif choice == 3:
        mul(input("Multiply this: "),input("by this: "))
    elif choice == 4:
        div(input("Divide this: "),input("by this: "))
    elif choice == 5:
        loop = 0


print "Thankyou for using calculator.py!"

# NOW THE PROGRAM REALLY FINISHES

Answer 1

Your problem is that you are using input() which (in Python 2.x) parses the input as Python code, meaning this will only work as intended if the user enters a string surrounded in quotes.

You probably want raw_input() which gives you a string (as input() does in 3.x), and is more secure as it doesn’t allow the execution of arbitrary code. Note that it will then be up to you to try to convert this to a number, and catch the exception if that fails, telling you the user hasn’t entered a number.

Type checking is frowned upon in Python, as it arbitrarily restricts classes – something which Python doesn’t normally do as it’s duck typed – it’s better to try to do what you want to do, and catch the exception if it fails, handling it gracefully. This is known as asking for forgiveness, not permission.