Python: Metaclass properties override class attributes, sometimes?

Question

The result of the below code boggles me:

class MyClass(type):
    @property
    def a(self):
        return 1

class MyObject(object):
    __metaclass__ = MyClass

    a = 2

print MyObject.a
print object.__getattribute__(MyObject, 'a')
print type.__getattribute__(MyObject, 'a')
print MyObject.__dict__['a']
print MyObject().a

I really expect this to just print 2 repeatedly, but it prints 1 1 1 2 2. Is there a way this makes any intuitive sense?

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To clarify: I understand that this behavior is well documented (here, "data descriptors"), but I want to have an understanding of why this makes sense, and why the core devs implemented descriptors this way.

Answer 1

Properties are data descriptors; in attribute lookups, they take priority over identically-named entries in the dict of an instance of the class with the property. That means that with

MyObject.a

the a property in MyClass takes priority over the a entry in MyObject's dict. Similarly,

object.__getattribute__(MyObject, 'a')
type.__getattribute__(MyObject, 'a')

object.__getattribute__ and type.__getattribute__ both respect the priority of data descriptors over instance dict entries, so the property wins.

On the other hand,

MyObject.__dict__['a']

this explicitly does a dict lookup. It only sees things in MyObject's dict, ignoring the normal attribute lookup mechanisms.

For the last line:

MyObject().a

the MyClass descriptor only applies to instances of MyClass, not instances of its instances. The attribute lookup mechanism doesn't see the property.