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adding methods on the fly (was Re: Smalltalk and Python)

Alex Martelli aleaxit at yahoo.com
Mon Dec 18 04:12:42 EST 2000 
[Note: we've already been kindly asked to stop posting
this thread to fj.comp.lang.ruby, using comp.lang.ruby
instead, so I edited the newsgroups line accordingly]

"Greg Ewing" <greg at cosc.canterbury.ac.nz> wrote in message
news:3A3D8815.801E773D at cosc.canterbury.ac.nz...
> Andrew Dalke wrote:
> >
> > >>> bed = make_a(Furniture, Surface, Scenery)
>
> But how do I give it methods?

Most easily, actually -- in any of several handy ways.


>   bed(Furniture, Surface, Scenery):
>
>     def lie_on(self):
>       print "It's so comfy, you fall instantly" \
>         " asleep, and dream of a golden key sitting" \
>         " on a shelf in the pantry."

Using my version of make_a, i.e., to recap:


import new

def make_a(*classes, **attribs):
    return new.classobj('blob', classes, attribs)()

class Furniture:
     def comfy(self): return 1

class Surface:
    def genus(self): return 0  # Imagine a spherical cow...

class Scenery:
    def description(self): return "boring"

bed = make_a(Furniture, Surface, Scenery)


you have a range of choices, e.g., the most explicit:

def anewmethodjustforbed(self):
    print "elegant and comfy"
bed.lie_on = new.instancemethod(anewmethodjustforbed,
    bed, bed.__class__)


or, a tad less explicitly but perhaps more simply:

bed.__class__.lie_on = anewmethodjustforbed

which takes advantage of the implicit function->method
coercion [which happens when a reference to a function
is bound to a *class*'s attribute -- not an _instance_
attribute], and of course of the fact that instance 'bed'
is the sole instance of its class, so, in order to affect
that instance's behavior, we're at liberty to tweak its
class's:-).

It's just an issue of preferred style.  If one is using
the new module to build the class for object 'bed', I
think it might be more consistent to keep using the same
module also for building new methods to assign on the
fly; if one is using Andrew's original suggestion and
building that class less explicitly, then the implicit
coercion based approach might be better.


Note that my version of function make_a takes keyword
arguments and turns them into class-attributes for
the new class, so, if you already know what methods
you want to add to 'bed' as you generate it, another
alternative is to take advantage of that:

bed = make_a(Furniture, Surface, Scenery,
        lie_on=anewmethodjustforbed)


Of course you'll want to factor things appropriately.
If you have many methods that just print out stuff
while ignoring self, or using self just for read
access to instance data attributes, etc, you might
build a function encapsulating that:

def printout(self, message):
    print message % self.__dict__

and use it to build all on-the-fly methods of this
kind:

bed = make_a(Furniture, Surface, Scenery,
        lie_on=lambda self: printout(self, "whatever"))

or you might prefer a closure-factory approach,
rather than the lambda approach:

def printout_maker(message):
    def printout(self, message=message):
        print message % self.__dict__
    return printout

bed = make_a(Furniture, Surface, Scenery,
        lie_on=printout_maker("whatever"))

or, of course, the closure-factory could also use
module new rather than the implicit approach I've
taken here (which strictly parallel's Andrew's
original suggestion for make_a, except that here
we're using a local function rather than a local
class as in his case).


I think that some one out of these approaches, or
some variant thereof, will probably meet all kinds
of "how do I add a method" requirements.


Alex
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