Type-checking declarative attributes via metaclass shenanigans

metaclass-fields.py

import six


class TypedField(object):
    def __init__(self, classinfo):
        self._classinfo = classinfo
        self.fieldname = None  # Set by the metaclass

    def __get__(self, instance, owner):
        try:
            return instance._fieldvalues[self.fieldname]
        except KeyError:
            raise AttributeError('Attribute {0!r} is not set'.format(
                self.fieldname
            ))

    def __set__(self, instance, value):
        if not isinstance(value, self._classinfo):
            raise TypeError('Invalid type {0!r} for attribute {1!r}'.format(
                type(value), self.fieldname,
            ))
        instance._fieldvalues[self.fieldname] = value

    def __delete__(self, instance):
        try:
            del instance._fieldvalues[self.fieldname]
        except KeyError:
            raise AttributeError('Attribute {0!r} is not set'.format(
                self.fieldname
            ))


class FieldsMeta(type):
    def __new__(metacls, classname, bases, classdict):
        for attrname, attrvalue in classdict.items():
            if isinstance(attrvalue, TypedField):
                attrvalue.fieldname = attrname
        return super(FieldsMeta, metacls).__new__(
            metacls, classname, bases, classdict)


class ModelBase(six.with_metaclass(FieldsMeta, object)):
    def __init__(self):
        self._fieldvalues = {}


class MyModel(ModelBase):
    name = TypedField(str)
    index = TypedField(int)


import unittest


class ShenanigansTestCase(unittest.TestCase):
    def test_setting_and_getting(self):
        m = MyModel()
        m.name = 'Alice'
        m.index = 1
        self.assertEqual(m.name, 'Alice')
        self.assertEqual(m.index, 1)

    def test_setting_checks_type(self):
        m = MyModel()
        with self.assertRaises(TypeError) as excinfo:
            m.index = 'not an int'
        self.assertIn(
            'Invalid type {0!r}'.format(str),
            str(excinfo.exception)
        )