从内置类型派生时如何设置“the”值？

Question

I'm (trying) to do some meta programming and things seem to be working until I try to do a set.

class MetaField(type):

    def __new__(meta, clsname, bases, attrs):
        bases = (Field,) + bases
        return super(MetaField, meta).__new__(meta, clsname, bases, attrs)

    def __call__(cls, *args, **kw):
        cls._primary_key = kw.pop('primary_key', False)
        return super(MetaField, cls).__call__(*args, **kw)

class Field(object):

    def __new__(cls, *args, **kw):
        return super(Field, cls).__new__(cls, *args)

    #def __set__(self, instance, value):
    #    magic needs to go here

    #def __get__(self, instance, owner):
    #    more magic here

    @property
    def primary_key(self):
        return self._primary_key

class IntField(int):
    __metaclass__ = MetaField

class SomeObject(object):
    # trying with descriptors here
    i = IntField(10, primary_key=True, foo='set')


obj = SomeObject()
assert obj.i == 10
assert obj.i.primary_key == True

obj.i = 11
assert obj.i == 11

assert obj.i.primary_key == True
AttributeError: 'int' object has no attribute 'primary_key'

Which makes sense since obj.i is no longer a IntField but a plain int .

So in Field.__set__ if I could do something along the lines of super(self) = value but have self still be a IntField that'd be super duper.

I've tried to implement a ProxyFieldDescriptor as described at http://blog.kevinastone.com/django-model-descriptors.html but when you return from __get__ you loose all your supplementary info like _primary_key .

How do you set the actual value of int from within a derived class?

Answer 1

First and foremost, you should probably burn in developer hell if you ever do this on production code. Secondly, here's what you're trying to do:

class MetaField(type):
    def __new__(meta, clsname, bases, attrs):
        bases = (Field,) + bases
        return super(MetaField, meta).__new__(meta, clsname, bases, attrs)

    def __call__(cls, *args, **kw):
        cls._primary_key = kw.pop('primary_key', False)
        cls._name = kw.pop('name')
        return super(MetaField, cls).__call__(*args, **kw)


class Field(object):

    def __getattr__(self, item):
        if item == 'primary_key':
            return self._primary_key
        else:
            return self.__dict__[item]

    def __set__(self, instance, value):
        instance.__dict__[self._name] = IntField(value, primary_key=self.primary_key, name=self._name)


class IntField(int):
    __metaclass__ = MetaField


class SomeObject(object):
    # trying with descriptors here
    i = IntField(10, primary_key=True, name='i')


if __name__ == '__main__':
    obj = SomeObject()
    assert obj.i == 10
    assert obj.i.primary_key == True

    obj.i = 11
    assert obj.i == 11

    assert obj.i.primary_key == True
# g2g

The key here is, as is often the case with descriptors, you need to name them. If you don't name them, there's no way to access the instance variable in the event you need to set/get with it. If that's not an issue, you don't need a name. But in this case you do. I made the necessary adjustments.

Although I'm sure you're well aware of this there are way easier ways to do what you're trying to accomplish... this is the path of the dark side.

Answer 2

This is a job for the __init__ method. It acts as a constructor and is called when your class is instantiated as an object.

  class A:
    def __init__(self, x, y):
      self.x = x
      self.y = y

  a = A(5, 6)
  assert a.x == 5 and a.y == 6

The way you were defining things, those were class attributes and were not associated with particular instances.