Question

When I enter a letter for one of the numbers I get an error:

    Traceback (most recent call last):
      File "/Users/rodchar/Documents/Hello World Katie/testing.py", line 7, in <module>
        x = input("What is your first number?")
      File "<string>", line 1, in <module>
    NameError: name 's' is not defined



x = 0
y = 0

isValid = False

while isValid == False:
    x = input("What is your first number?")

    try:
        float(x)
        isValid = True
    except:
        isValid = False

    y = input("What is your second number?")

    try:
        float(y)
        isValid = True
    except:
        isValid = False


print "The answer is: %s" % (x+y)

Answer 1

Firstly, at least in Python 2.7, input() is not what you want for getting user input. You actually want raw_input(). (This is, indeed, confusing, and has been fixed in Python 3.)

Secondly, as @jamylak said, the statement giving the exception is not inside a try/catch block.

Thirdly, when you catch exceptions using except:, you should really be catching a specific type of exception (except ValueError:) instead of any and all exceptions. Catching all exceptions is bad because it masks bugs that raise exceptions you didn’t anticipate.

---

A better way to write this would be:

• Apply the “Don’t Repeat Yourself” principle (DRY) – the code for getting x and getting y from the user is very similar. Instead of writing the same thing twice, write it once and reuse it.
• Wrap the “get a float from the user” functionality up in a named function.
• Catch only the specific type of exception that you are prepared to handle.

Tested on Python 2.7:

def get_float_from_user (prompt):
    while True:
        try:
            user_input = float(raw_input(prompt))
            return user_input
        except ValueError:
            print ("Not a floating point number. Please try again.")

x = get_float_from_user("Enter x.")
y = get_float_from_user("Enter y.")

print (x+y)