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[Python-Dev] Non-string keys in type dict

PJ Eby pje at telecommunity.com
Thu Mar 8 18:16:28 CET 2012 
On Thu, Mar 8, 2012 at 2:43 AM, Ethan Furman <ethan at stoneleaf.us> wrote:
>
> PJ Eby wrote:
>>
>> Short version: AddOns are things you can use to dynamically extend
instances -- a bit like the "decorator" in "decorator pattern" (not to be
confused with Python decorators).  Rather than synthesize a unique string
as a dictionary key, I just used the AddOn classes themselves as keys.
 This works fine for object instances, but gets hairy once classes come
into play.
>
>
> Are you able to modify classes after class creation in Python 3? Without
using a metaclass?

For ClassAddOns, it really doesn't matter; you can't remove them from the
class they attach to.  Addons created after the class is finalized use a
weakref dictionary to attach to their classes.

Now that I've gone back and looked at the code, the only reason that
ClassAddOns even use the class __dict__ in the first place is because it's
a convenient place to put them while the class is being built.  With only
slightly hairier code, I could use an __addons__ dict in the class
namespace while it's being built, but there'll then be a performance hit at
look up time to do cls.__dict__['__addons__'][key] instead of
cls.__dict__[key].

Actually, now that I'm thinking about it, the non-modifiability of class
dictionaries is actually a feature for this use case: if I make an
__addons__ dict, that dict is mutable.  That means I'll have to move to
string keys or have some sort of immutable dict type available...  ;-)
 (Either that, or do some other, more complex refactoring.)
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