Trying to reimplement SQLAlchemy's query.filter magic

filter_magic.py

# after using SQLAlchemy I've decided to try my python skills in coding it's filter function magic
# without looking into their source code, just for fun
# Note: it's not complete, just proof of concept

class BindProperty(type):
    def __new__(cls, name, bases, namespace, **kwds):
        result = type.__new__(cls, name, bases, dict(namespace))

        for name, value in namespace.items():
            if isinstance(value, Property):
                value._update_attribute_name(name)

        return result


class Condition(object):

    def __init__(self, property_name):
        self.property_name = property_name

    def __gt__(self, other):
        return lambda instance: getattr(instance, self.property_name) > other

    def __eq__(self, other):
        return lambda instance: getattr(instance, self.property_name) == other


class Property(object):

    def _update_attribute_name(self, property_name):
        self.instance_attribute_name = '_%s' % property_name
        self.condition_checker = Condition(property_name)

    def __get__(self, instance, owner):
        if instance is None:
            return self.condition_checker
        else:
            return getattr(instance, self.instance_attribute_name, None)

    def __set__(self, instance, value):
        setattr(instance, self.instance_attribute_name, value)


class User(object, metaclass=BindProperty):
    name = Property()
    age = Property()

    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __repr__(self):
        return '<User(name="%s", age=%s)>' % (self.name, self.age)


def where(items, *conditions):
    return [
        item for item in items
        if all([c(item) for c in conditions])
    ]


users = (
    User('Alex', 19),
    User('John', 21),
    User('Mark', 25),
    User('John', 15),
)
print('Source:', users)

filtered1 = where(users, User.name == 'John')
print('filtered1:', filtered1)

filtered2 = where(users, User.age > 18)
print('filtered2:', filtered2)

filtered3 = where(users, User.name == 'John', User.age > 20)
print('filtered3:', filtered3)