[Tutor] validation

[John Fouhy]

On 27/08/07, Michael <python at mrfab.info> wrote:
> I am fairly new to Python and I wish to validate input. Such as wanting
> to check and make sure that an integer is entered and the program not
> crashing when a naughty user enters a character instead. I have been
> trying to use the Type() function but I cannot work out how to check the
> return value? Caomparing it to 'int' or 'str' isn't working, or should I
> be using the isinstance property? I want to use raw_input and check that
> it is a number. Can someone point me in the right direction?

Hi Michael,

raw_input() will give you a string.  Python does not do automatic type
conversions, like some languages do.  To convert string to int, you
can use int().  e.g.:

  user_input = raw_input()
  num = int(user_input)

If you call int() on something that is not a valid integer string,
this will throw an exception -- TypeError, I think.  So the pythonic
way to check is to catch the exception:

while True:
  user_input = raw_input()
  try:
    num = int(user_input)
    break
  except TypeError:
    print 'Oops, try again!'

HTH!

-- 
John.