[Python-3000] rethinking pep 3115

[Nick Coghlan]

Alex Martelli wrote:
>> (2)
>> the second-best solution i could think of is just passing the dict as a
>> keyword argument to the class, like so:
>>
>> class Spam(metaclass = Bacon, dict = {}):
>>     ...
>>
>> so you could explicitly state you need a special dict.
> 
> I like this one, with classdict being the keyword (dict is the name of
> a builtin type and we shouldn't encourage the frequent but iffy
> practice of 'overriding' builtin identifiers).

So instead of being able to write:

   class MyStruct(Struct):
      first = 1
      second = 2
      third = 3

everyone defining a Struct subclass has to write:

   class MyStruct(Struct, classdict=OrderedDict()):
      first = 1
      second = 2
      third = 3

Forgive my confusion, but exactly *how* is that meant to be an improvement?

The use of a special ordered dictionary should be an internal 
implementation detail of the Struct class, and PEP 3115 makes it exactly 
that. The PEP's approach means that simple cases, while possibly being 
slightly harder to write, will 'just work' when it comes time to use 
them, while more complicated cases involving multiple metaclasses should 
still be possible.

I will also note that the PEP allows someone to write their own base 
class which accepts the 'classdict' keyword argument if they so choose.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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