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