bruno modulix wrote:
Mike C. Fletcher wrote:
(snip)
Though the don't go into extreme detail on decorators (they are
basically syntactic sugar for a particular type of descriptor).
Err... Could you elaborate on this ? Decorators are syntactic sugar for
function wrapping, while descriptors are a 'protocol' to hook into
attribute lookup, so I don't really see how one can describe the first
in terms of the second...
There are two major types of descriptors, the elven and the dwarven.
* Elven descriptors are things like property, BasicProperty, VRML97
fields, Zope Field Properties, or PEAK's mechanisms. They mediate
access to an instance's attributes, (both setting and getting of
values) and are used primarily for domain modeling.
* Dwarven descriptors are things like staticmethod or classmethod;
function-like things that tend to look like a function and quack
like a function, but have some special property or functionality
attached when accessed as an attribute of a class/instance
(functions themselves return instance methods or class methods
depending on how they are retrieved).
As you'll read in the PyCon 2005 paper pointed to, the whole set
basically grew out of a generalisation of what functions were already
doing (conceptually). Functions are, in short, the proto-descriptor.
That is, objects which control what result is returned by attribute
access (e.g. an instance or unbound instance method). Many (most?)
dwarven descriptors are implemented as simple wrapper functions around
the base function to use the function's already-present descriptor hooks.
Decorators are syntactic sugar for defining dwarven descriptors. In
Python 2.2 and 2.3 a dwarven descriptor was instantiated like this:
def function( cls ):
"""Do something simple with the class"""
function = classmethod( function )
in Python 2.4+, they can be instantiated like this:
@classmethod
def function( cls ):
"""Do something simple with the class"""
technically you *could* return something that's not a descriptor from
the decorator (e.g. classmethod), but it's unlikely you'd do that very
often.
HTH,
Mike
--
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Mike C. Fletcher
Designer, VR Plumber, Coder
http://www.vrplumber.com http://blog.vrplumber.com 
