[Python-3000] Fwd: proposal: disambiguating type

[Nick Coghlan]

Guido van Rossum wrote:
> On 5/22/06, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
>> tomer filiba wrote:
>>
>>> i suggest splitting this overloaded meaning into two separate builtins:
>>> * type(name, bases, dict) - a factory for types
>>> * typeof(obj) - returns the type of the object
>> Or just drop the function usage altogether and make
>> __class__ the one obvious way to find out something's
>> class/type.
> 
> Well, you could overload __class__ to "lie" -- but type won't. I'd
> rather not lost that functionality. I expect that with proxies
> becoming more popular they may start lying about __class__. For most
> purposes that's fine but I'd like to be able to tell whether I'm
> dealing with a proxy, if I really need to know.

That suggests to me that Tomer's on the right track in renaming the query 
function and leaving the metaclass alone. A lot of the code that currently 
uses type() directly doesn't work properly with classic classes and other 
metaclasses (like remote proxies) that persuade "__class__" to lie (I don't 
use classic classes if I can help it, so I'm fairly sure some of my own code 
fits into this category).

You could even standardise the "use __class__ if it's present and typeof(x) 
otherwise" pattern as a separate query function:

def classof(instance):
     try:
         return instance.__class__
     except AttributeError:
         return typeof(instance)

Then:

   type = the standard metaclass
   typeof = query function that an instance's metaclass cannot affect
   classof = query function that an instance's metaclass can affect

Metaclasses that play games with __class__ (like types.ClassType) can then be 
detected by the fact that typeof(x) != classof(x).

classof and typeof could actually be added in 2.x - it's only the removal of 
type's single argument behaviour that would have to wait until Py3k (or do the 
deprecation dance in the 2.x series)

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
---------------------------------------------------------------
             http://www.boredomandlaziness.org