Having a simple Python class like this:

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

Which is the correct approach to check these 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?
6....

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