[Python-3000] PEP 3124 - Overloading, Generic Functions, Interfaces, etc.
Phillip J. Eby
pje at telecommunity.com
Thu May 10 03:23:41 CEST 2007
At 06:26 PM 5/9/2007 -0400, Jim Jewett wrote:
>On 5/9/07, Phillip J. Eby <pje at telecommunity.com> wrote:
>>At 09:54 PM 5/9/2007 +0200, BJörn Lindqvist wrote:
>> >What if they have defined a do_stuff that dispatch on ClassC that is a
>> >subclass of ClassA? Good luck in figuring out what the code does.
>> >With the non-overloaded version you also have the ability to insert
>> >debug print statements to figure out what happens.
>> def debug_it(ob: ClassC):
>> import pdb
>I think this may be backwards from his question. As I read it, you
>know about class A, but have never heard about class C (which happens
>to be a substitute for A). Someone added a different do_stuff
>implementation for class C.
> def debug_it(obj: ClassA): # Never called, it is a classC
Actually, if you read what was said above, ClassC is a subclass of
ClassA, so the above *is* called.
> def debug_it(obj: not ClassA) # can't do this?
Actually, you can, if you create something like a NotClass type and
register methods to define its implication relationships to classes
and other criteria. Of course, it then wouldn't be called for ClassC...
> def debug_it(obj): # OK, trace *everything*.
> # Or, at least, everything that nicely did a call_next_method,
> # in case you wanted to wrap it this way. Objects that thought
> # they were providing a complete concrete implementation will
> # still sneak through
Which is an excellent demonstration, by the way, of another reason
why before/after methods are useful. They're all *always* called
before and after the primary methods, regardless of how many of them
> def wrap_the_generic(generic_name, debug_it):
> orig = generic_name
> def replacement( ...) # hope you get the .sig right
> generic_name = replacement # hope you can monkeypatch
> # uhh ... was the original supposed to have additional behavior,
> # for more registrations, etc...
I don't understand what this last example is supposed to be, but note
that if you want to create special @debug methods with higher
precedence than Around methods, it's relatively simple:
"""Like an Around, but with higher precedence"""
debug = Debug.make_decorator('debug')
(It occurs to me that although the current prototype implementation
requires you to explicitly declare method override relationships for
all applicable types, I should probably make the transitive
declaration(s) automatic, so that the above would require only
'always_overrides(Debug, Around)' to work.)
>Unless I'm missing something, this only simplifies things when all
>specific implementations not only drink the kool-ade, but avoid
>kool-ade related bugs.
I don't understand what you mean here.
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