Python: input accepts integer but crashes on a string

Question

For a practice problem in my homework, i'm making a guessing game that starts off by asking for a number. I'm trying to implement a way that prints "invalid input" when given a string, but i get an error message. here is my code:

def get_input():
    '''
    Continually prompt the user for a number, 1,2 or 3 until
    the user provides a good input. You will need a type conversion.
    :return: The users chosen number as an integer
    '''
    guess=int(input("give me 1,2,3"))
    while True:
        if guess==1 or guess==2 or guess==3:
            return guess
        else:
            print "Invalid input!"

        guess=int(input("give me 1,2,3"))

I get this message when I put in a string such as

hello/System/Library/Frameworks/Python.framework/Versions/2.6/bin/python2.6 /Users/bob/PycharmProjects/untitled/warmup/__init__.py

give me 1,2,3hello

Traceback (most recent call last):
  File "/Users/bob/PycharmProjects/untitled/warmup/__init__.py", line 51, in <module>
    get_input()
  File "/Users/bob/PycharmProjects/untitled/warmup/__init__.py", line 43, in get_input
    guess=int(input("give me 1,2,3"))
  File "<string>", line 1, in <module>
NameError: name 'hello' is not defined

Process finished with exit code 1

Answer 1

You need to use raw_input for python2, input tries to evaluate the string so it looks for a variable called name and errors as there is no variable called name defined anywhere. You should never use input in python2, it is equivalent to eval(raw_input()) which has obvious security risks.

So to spell it out more clearly, don't use input to take input from a user in python2, use raw_input() and in your case take the raw_input using a try/except catching a ValueError.

def get_input():
    '''
    Continually prompt the user for a number, 1,2 or 3 until
    the user provides a good input. You will need a type conversion.
    :return: The users chosen number as an integer
    '''

    while True:
        try:
            guess = int(raw_input("give me 1,2,3"))
            if guess in (1, 2, 3):
                return guess
        except ValueError:
            pass
        print("Invalid input!")

The fact you are just cheking for 1,2 or 3 means you could also just do the casting after you confirm:

def get_input():
    '''
    Continually prompt the user for a number, 1,2 or 3 until
    the user provides a good input. You will need a type conversion.
    :return: The users chosen number as an integer
    '''

    while True:
        guess = raw_input("give me 1,2,3")
        if guess in ("1", "2", "3"):
            return int(guess)
        print("Invalid input!")