Correct approach to validate attributes of an instance of class

Question

Having a simple Python class like this:

class Spam(object):     __init__(self, description, value):         self.description = description         self.value = value 

I would like to check the following constraints:

• "description cannot be empty"
• "value must be greater than zero"

Should I:
1. validate data before creating spam object ?
2. check data on __init__ method ?
3. create an is_valid method on Spam class and call it with spam.isValid() ?
4. create an is_valid static method on Spam class and call it with Spam.isValid(description, value) ?
5. check data on setters declaration ?
6. etc.

Could you recommend a well designed/Pythonic/not verbose (on class with many attributes)/elegant approach?

Answer 1

You can use Python properties to cleanly apply rules to each field separately, and enforce them even when client code tries to change the field:

class Spam(object):     def __init__(self, description, value):         self.description = description         self.value = value      @property     def description(self):         return self._description      @description.setter     def description(self, d):         if not d: raise Exception("description cannot be empty")         self._description = d      @property     def value(self):         return self._value      @value.setter     def value(self, v):         if not (v > 0): raise Exception("value must be greater than zero")         self._value = v 

An exception will be thrown on any attempt to violate the rules, even in the __init__ function, in which case object construction will fail.

UPDATE: Sometime between 2010 and now, I learned about operator.attrgetter:

import operator  class Spam(object):     def __init__(self, description, value):         self.description = description         self.value = value      description = property(operator.attrgetter('_description'))      @description.setter     def description(self, d):         if not d: raise Exception("description cannot be empty")         self._description = d      value = property(operator.attrgetter('_value'))      @value.setter     def value(self, v):         if not (v > 0): raise Exception("value must be greater than zero")         self._value = v 

Answer 2

If you only want to validate the values when the object is created AND passing in invalid values is considered a programming error then I would use assertions:

class Spam(object):     def __init__(self, description, value):         assert description != ""         assert value > 0         self.description = description         self.value = value 

This is about as concise as you are going to get, and clearly documents that these are preconditions for creating the object.