[Python-Dev] method decorators (PEP 318) - decorators vs transforms

[Jewett, Jim J]

Guido (based on discussion with Robert Mollitor and Jim Hugunin)
wrote that there are two very different use cases.

We have been discussing transformers, which change how/what the
function does.  But many uses (such as "release" from the PEP)
are true decorators, which are really closer to:

   def foo():
      pass
   foo.attr = val

> For (2) I am thinking aloud here:

>   def foobar(self, arg):
>       @author: "Guido van Rossum"
>       @deprecated
>       pass

If I understand, the grammar production would be

	'@'+identifier [':' expression]

with expression defaulting to True.

Then

    def foobar as [classmethod, synchronized(lock23)] (args):
        @author: "Guido van Rossum"
        @version: (2,3)
        @deprecated
        @variant: @version + (@author,)

        # this next one might be asking for trouble
        @x_starts_as: @x

        @magic_number: random.randint(34) + y
        @version: @version + ("alpha", 1)

        "This is an example function"
        x = 5
        return x


would be equivalent to:

    def foobar(args):
        "This is an example function"
        x = 5
        return x

    # Evaluate the transforms first, in case they return a 
    # different object, which would not have the annotations.
    foobar = classmethod(foobar)
    foobar = synchronized(lock23)(foobar)

    # Now the annotations go on the final object.  Do we
    # need to worry about immutable objects, or is that the 
    # programmer's own fault?
    foobar.author = "Guido van Rossum"
    foobar.version = (2,3)

    # Default value is True
    foobar.deprecated = True

    # Do decorators have access to current attributes?
    # Did '@' read better than 'self' or 'this'?
    foobar.variant = foobar.version + (foobar.author,)

    # Does the attribute have access to the local scope?
    foobar.x_starts_as = foobar.x

    # Do decorators have access to the enclosing scope? 
    foobar.magic_number = random.randint(34) + y

    # Decorators don't really have to commute, do they?
    foobar.version = foobar.version + ("alpha", 1)

-jJ