Question

I’ve been successfully using Python properties, 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?

Answer 1

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.

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