class Foo(BaseClass):
    #Определение класса
    pass
#Где-то в другом модуле..
from myapp.models import Foo
x = Foo.objects.get_all() # Возвращается итератор
for item in x:
    #Перебираем объекты
    pass

class classproperty(object):
    def __init__(self, fget):
        self.fget = fget
    def __get__(self, owner, cls):
        if owner is not None:
            raise AttributeError("objects is class only")
        return self.fget(cls)
class Field(object):
    def __init__(self, **kwargs):
        self.null = kwargs.get("null", False)
    def _set_value(self, value):
        if value is None:
            if not self.null:
                raise ValueError("Field %s IS_NULL=FALSE" % self.__class__.__name__)
            self.value = None
            return
        try:
            self.value = self.validate(value)
        except:
            raise ValueError("Invalid value")
        return self
    def validate(self, value):
        pass
class ModelMeta(type):
    def __new__(cls, name, bases, dct):
        fields = dict()
        for attr_name, value in dct.iteritems():
            if isinstance(value, Field):
                fields[attr_name] = value
        dct['_fields'] = fields
        return type.__new__(cls, name, bases, dct)
db = dict(mymodel=[
                dict(id=1, a=100, b=3),
                dict(id=2, a=100, b=None),
                dict(id=3, a=3, b=None)
    ])
class Manager(object):
    def __init__(self, model_class, **kwargs):
        self.model_class = model_class
        self.where = kwargs.get("where") or dict()
        self.db_model_name = model_class.__name__.lower()
    def get(self, **kwargs):
        data = iter(db[self.db_model_name])
        for field_name, value in kwargs.iteritems():
            data = (x for x in data if x[field_name] == value)
        return self.model_class(**list(data)[0])
    def filter(self, **kwargs):
        if not kwargs:
            raise AttributeError("Empty args")
        self.where.update(kwargs)
        return self
    def all(self):
        return Manager(self.model_class, where=self.where)
    def __str__(self):
        return str(self.execute())
    def __iter__(self):
        return self.execute()
    def execute(self):
        data = db[self.db_model_name]
        for field_name, value in self.where.iteritems():
            data = (x for x in data if x[field_name] == value)
        return (self.model_class(**item) for item in data)
class Model(object):
    __metaclass__ = ModelMeta
    def __init__(self, **kwargs):
        setattr(self, "id", kwargs.get("id"))
        for field_name, field_class in self._fields.iteritems():
            value = kwargs.get(field_name)
            setattr(self, field_name, self._fields[field_name]._set_value(value))
    @classproperty
    def objects(cls):
        return Manager(cls)
class IntegerField(Field):
    def validate(self, value):
        return int(value)
    def __str__(self):
        return str(self.value)
class MyModel(Model):
    a = IntegerField()
    b = IntegerField(null=True)
print MyModel.objects.get(a=3)
for x in MyModel.objects.all():
    print x.a, x.id
print MyModel(a=1)