[Python-Dev] PEP318 metaclass approach

[Sean Ross]

Hi.
I've posted this code to python-list, but I thought I might post it 
here as well. This is a metaclass approach to dealing with the issues 
PEP318 is meant to address. It's a quick hack that allows you to put 
the method decoration and annotation information before the method 
definition. I just wanted to see if this could be done without changing 
the language syntax. Looks like it's atleast a possibility, though this 
only works with methods, and there are likly other issues I'm not aware 
of. Anyway, here's the code:



import sys

def delayed_decorate(method, *decorators, **attributes):
    "Decorate method during class instantation"
    method.__dict__.update(attributes)
    for decorator in decorators:
        method = decorator(method)
    return method

def decorate(funcname, *decorators, **attributes):
    "Mark method for delayed decoration"
    clsdict = sys._getframe(1).f_locals
    clsdict.setdefault("decorated", {})[funcname] = \
        (decorators, attributes)


class MetaDecorate(type):
    "Enables delayed decoration and annotation of methods"
    def __new__(klass, name, bases, _dict):
        if "decorated" in _dict:
            for m, (d, a) in _dict["decorated"].iteritems():
                _dict[m] = delayed_decorate(_dict[m], *d, **a)
            del _dict["decorated"]
        return type.__new__(klass, name, bases, _dict)
    
class Decorated(object):
    __metaclass__ = MetaDecorate

class C(Decorated):
    a = "A"

    decorate("f", classmethod, author="Sean Ross", version="0.9.2")
    def f(klass):
        return klass.a
    
    decorate("g", staticmethod)
    def g():
        return "G"
    
print "C.f.author =", C.f.author
print "C.f.version =", C.f.version
print "C.f() =>", C.f()
print "C.g() =>", C.g()