[Python-Dev] Class decorators (original) (raw)

Jack Diederich jack at performancedrivers.com
Thu Mar 30 03:00:59 CEST 2006

On Wed, Mar 29, 2006 at 07:23:03PM -0500, Phillip J. Eby wrote:

At 11:07 AM 3/29/2006 -0800, Guido van Rossum wrote: >On 3/28/06, Phillip J. Eby <pje at telecommunity.com> wrote: > > If we're using Zope 3 as an example, I personally find that: > > > > class Foo: > > """Docstring here, blah blah blah > > """ > > implements(IFoo) > > > > is easier to read than: > > > > @implements(IFoo) > > class Foo: > > """Docstring here, blah blah blah > > """ > >But the former also smells more of magic.

My comment above was only about readable placement of the decorators, not the actual syntax. Many approaches to the actual syntax in the body are possible. For example, what did you think of Fred Drakes's "@class" proposal? To specify it formally, one could say that this: @class EXPR in a class scope would expand to the equivalent of: locals().setdefault('decorators',[]).append(EXPR) and is a syntax error if placed anywhere else. That, combined with support for processing decorators at class creation time, would fulfill the desired semantics without any implicit "magic".

A function decorator takes a function as an argument and returns something (probably a function and maybe even the very same function). This is exactly what class decorators should do or we should call them something else and give them a distinct syntax.

A function decorator is there to replace code like:

def myfunc(a, b, c):

half a screen of code

myfunc = mangle(myfunc)

Likewise class decorators would save me from typing

class MyClass:

many functions taking half a screen of code each

register(MyClass, db_id=20)

I used to do this with metaclasses but stopped because it requires making 'db_id' a member of the class which is magically noticed by a metaclass two files away. Using metaclasses also required gross hacks like checking for a 'DO_NOT_REGISTER' member for subclasses that wanted to inherit from a class that had a Register metaclass but didn't want to be registered. Yuck.

If you want to do lots of Zopeish stuff mostly inside the class write a decorator that looks for it in the class body.

@zope.find_magic_in_attr('mymagic') class MyClass: mymagic = [] # some long hairy thing

Talking about something other than a decorator or proposing all new syntax is just going to get this pronounced out of existence.

If-I-wanted-zope-I'd-use-zope-ly,

-jackdied

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