How do Python properties work?

I've been successfully using Python propertie开发者_运维知识库s, but I don't see how they could work. If I dereference a property outside of a class, I just get an object of type property:

@property
def hello(): return "Hello, world!"

hello  # <property object at 0x9870a8>

But if I put a property in a class, the behavior is very different:

class Foo(object):
   @property
   def hello(self): return "Hello, world!"

Foo().hello # 'Hello, world!'

I've noticed that unbound Foo.hello is still the property object, so class instantiation must be doing the magic, but what magic is that?

As others have noted, they use a language feature called descriptors.

The reason that the actual property object is returned when you access it via a class Foo.hello lies in how the property implements the __get__(self, instance, owner) special method:

• If a descriptor is accessed on an instance, then that instance is passed as the appropriate argument, and owner is the class of that instance.
• When it is accessed through the class, then instance is None and only owner is passed. The property object recognizes this and returns self.

Besides the Descriptors howto, see also the documentation on Implementing Descriptors and Invoking Descriptors in the Language Guide.

In order for @properties to work properly the class needs to be a subclass of object. when the class is not a subclass of object then the first time you try access the setter it actually makes a new attribute with the shorter name instead of accessing through the setter.

The following does not work correctly.

class C(): # <-- Notice that object is missing

    def __init__(self):
        self._x = None

    @property
    def x(self):
        print 'getting value of x'
        return self._x

    @x.setter
    def x(self, x):
        print 'setting value of x'
        self._x = x

>>> c = C()
>>> c.x = 1
>>> print c.x, c._x
1 0

The following will work correctly

class C(object):

    def __init__(self):
        self._x = None

    @property
    def x(self):
        print 'getting value of x'
        return self._x

    @x.setter
    def x(self, x):
        print 'setting value of x'
        self._x = x

>>> c = C()
>>> c.x = 1
setting value of x
>>> print c.x, c._x
getting value of x
1 1

Properties are descriptors, and descriptors behave specially when member of a class instance. In short, if a is an instance of type A, and A.foo is a descriptor, then a.foo is equivalent to A.foo.__get__(a).

The property object just implements the descriptor protocol: http://docs.python.org/howto/descriptor.html