[Tutor] Most pythonic input validation

Robert Johansson robert.johansson at math.umu.se
Fri Oct 16 15:44:46 CEST 2009

> What if a valid user input has to be an integer between 10 and 20? I mean,
> it could be that you are doing something that only makes sense for that
> input. Would it be pythonic to add a test like:
> If prompt in range(10,21):

Better to write
  if 10 <= prompt <= 20:
>  ...
> else:
>  ... raise an error
> If I understand the error handling correctly you can add your own user
> defined error, but is that really what you should do in that case?

It depend on the context. You could raise ValueError (it's OK to raise
built-in exception types if they apply) or your own exception type or
just handle the error.


Thanks Kent,

If I wanted prompt to be an integer, is my check with range still bad? 

Is this the way to define my own error and to use it:

class MyValueError(Exception):
  define initialization and printing
  if test ok:
     raise MyValueError
except MyValueError:

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