[Compiler-sig] Re: [selfish-devel] Ugly hacks: modifying instance_getattr

[Tripp Lilley]

On Mon, 27 Nov 2000 echuck@mindspring.com wrote:

> You have lost me. Why do you want to hack on "obj.foo()"?

I refer you to our earlier discussion on selfish-devel, but to summarize
for others' benefit (or anguish, as the case may be):

	I have two slots, "scalar" and "method", which are, respectively,
	a scalar value and a method (bound using the "new" hacks). I want
	to access those slots according to the conventions established in
	the Self language, namely without regard to whether they're
	implemented as simple attributes or as methods. Thus:

		# retrieve

		o.scalar
		o.scalar( )

		o.method
		o.method( )


		# set

		o.scalar = 42
		o.scalar( 42 )

		o.method = 25
		o.method( 25 )


Taking first the case of the "scalar" slot:

	I want to define two attributes, one a simple value, one a method.
	When the slot "scalar" is retrieved as a simple attribute (ie:
	o.scalar), it will magically pop out of __dict__['scalar'] per
	normal Python getattr rules.

	On the other hand, when it's retrieved just prior to a
	CALL_FUNCTION bytecode (or by the PyObject_CallMethod call), it
	will try magically returning __dict__['__meth_scalar__'], a
	wrapper which handles "method" semantics for the slot. If that
	fails, it will try calling __getmethod__, and if that throws an
	AttributeError, it will fall back to normal semantics.

All of this is predicated on the setting of __use_getmethod__ or somesuch
in globals.

Now, the case of the "method" slot:

	When called as a simple scalar (ie: o.method), this would be
	trapped by either __getattr__, or by the new __attr_method__ hook
	proposed in PEP 213. Either of those would simply execute the
	method in-place, returning the resolved value.

	When called as a method, the contents of
	__dict__['__meth_method__'] would be returned, which happen to be
	a normal, old-fashioned method object, which is called as normal.

So, basically, it allows me to do away with the incredibly ugly
slot-wrapper crap I'm using right now to implement attr/method opacity.
For certain general cases, it's quite efficient. For other general cases,
the inefficiency is masked by other inefficiencies that aren't avoidable.
At least as far as I know right now :)

--
   Joy-Loving * Tripp Lilley  *  http://stargate.eheart.sg505.net/~tlilley/
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