Definition of "property"

[Eryk Sun]

On 5/30/21, Ethan Furman <ethan at stoneleaf.us> wrote:
>
>  > Properties are a special kind of attribute. Basically, when Python
> encounters the following code:
>  >
>  >     spam = SomeObject()
>  >     print(spam.eggs)
>  >
>  > it looks up eggs in spam, and then examines eggs to see if it has a
> __get__, __set__, or __delete__
>  > method — if it does, it's a property.

The above is not quite right. Having a __get__ method is not
sufficient. In the quoted example, the `eggs` attribute of the
SomeObject type has to be a data descriptor type, which means it
defines a __set__ and/or __delete__ method. A computed attribute
that's implemented by a data descriptor type cannot be overridden by
an instance attribute of the same name. In contrast, a non-data
descriptor type only defines a __get__ method  (e.g. the `function`
type is a non-data descriptor). A computed attribute that's
implemented by a non-data descriptor type will be overridden by an
instance attribute of the same name. The two common data descriptor
types are `property` and `member_descriptor` (from __slots__), but
creating custom data descriptor types is easy to implement.

See "customizing attribute access" in the data model documentation,
and in particular "implementing descriptors" and "invoking
descriptors":

https://docs.python.org/3/reference/datamodel.html#customizing-attribute-access