Most Pythonic way to do input validation [duplicate]

Question

What is the most "correct", Pythonic way to do user input validation in Python?

I've been using the following:

while True:
    stuff = input("Please enter foo: ")
    try:
        some_test(stuff)
        print("Thanks.")
        break
    except SomeException:
        print("Invalid input.")

Which is nice and readable, I suppose, but I can't help wondering if there isn't some built-in function or something I should be using instead.

Answer 1

I like decorators to separate the checking from the rest of the input handling.

#!/usr/bin/env python

def repeatOnError(*exceptions):
  def checking(function):
    def checked(*args, **kwargs):
      while True:
        try:
          result = function(*args, **kwargs)
        except exceptions as problem:
          print "There was a problem with the input:"
          print problem.__class__.__name__
          print problem
          print "Please repeat!"
        else: 
          return result
    return checked
  return checking

@repeatOnError(ValueError)
def getNumberOfIterations():
  return int(raw_input("Please enter the number of iterations: "))

iterationCounter = getNumberOfIterations()
print "You have chosen", iterationCounter, "iterations."

EDIT:

A decorator is more or less a wrapper for an existing function (or method). It takes the existing function (denoted below its @decorator directive) and returns a "replacement" for it. This replacement in our case calls the original function in a loop and catches any exception happening while doing so. If no exception happens, it just returns the result of the original function.

Answer 2

The most Pythonic way to do this kind of validation of "User INput" is to catch an appropriate exception.

Example:

def get_user_input():
    while True:
        try:
            return int(input("Please enter a number: "))
        except ValueError:
            print("Invalid input. Please try again!")

n = get_user_input()
print("Thanks! You entered: {0:d}".format(n))

It's also good practice to allow exceptions occur where they lie and allow them to bubble up rather than hide them so you can clearly see what's going wrong in a Python Traceback.

In this case Validating User Input -- Use Python's Duck Typing and catch the error. i.e: If it acts like a duct, it must be a duck. (If it acts like an int, it must be an int).