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

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 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.

Answer 2

if you want to only validate those values passed to the constructor, you could do:

class Spam(object):
    def __init__(self, description, value):
        if not description or value <=0:
            raise ValueError
        self.description = description
        self.value = value

This will of course will not prevent anyone from doing something like this:

>>> s = Spam('s', 5)
>>> s.value = 0
>>> s.value
0

So, correct approach depends on what you're trying to accomplish.

Answer 3

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):
    __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.

Answer 4

Unless you're hellbent on rolling your own, you can simply use formencode. It really shines with many attributes and schemas (just subclass schemas) and has a lot of useful validators builtin. As you can see this is the "validate data before creating spam object" approach.

from formencode import Schema, validators

class SpamSchema(Schema):
    description = validators.String(not_empty=True)
    value = validators.Int(min=0)

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

## how you actually validate depends on your application
def validate_input( cls, schema, **input):
    data = schema.to_python(input) # validate `input` dict with the schema
    return cls(**data) # it validated here, else there was an exception

# returns a Spam object
validate_input( Spam, SpamSchema, description='this works', value=5) 

# raises an exception with all the invalid fields
validate_input( Spam, SpamSchema, description='', value=-1) 

You could do the checks during __init__ too (and make them completely transparent with descriptors|decorators|metaclass), but I'm not a big fan of that. I like a clean barrier between user input and internal objects.