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To: python-list@python.org
From: Terry Jan Reedy <tjreedy@udel.edu>
Subject: Re: name lookup failure using metaclasses with unittests
Date: Fri, 12 Apr 2013 10:58:01 -0400
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On 4/12/2013 3:17 AM, Ulrich Eckhardt wrote:
> Am 11.04.2013 10:19, schrieb Steven D'Aprano:
>> if sys.version >= '3':
>
> Use sys.version_info >= (3,), otherwise your code breaks when upgrading
> to Python 10 and greater. ;^)
>
>
>>> The second question that came up was if there is a way to keep a
>>> metaclass defined inside the class or if the only way is to provide it
>>> externally. [...]
>>
>> Not in general, since the metaclass has to exist independently of the
>> class.
>
> Thanks for your explanations, they are appreciated.
>
>
>  > The class is an instance of your metaclass. That means that the
>  > metaclass must exist first, so it can be instantiated when you
>  > define the class.
>
> I don't like the approach to define the code to post-process a class
> before defining the class. It's a bit like top-posting, it messes up the
> reading order. Since I really intend to post-process the class, it seems
> that metaclasses are simply not the right tool.

Using a post-processing object as a metaclass or decorator necessarily 
requires predefinition. Such objects are usually used more than once.

For one-off postprocessing, I probably would not bother.

> At the moment, this leaves me with two options:
>
> 1. post-process the class
>
> class X:
>      pass
> # attach constants to clas X
> for i in (1, 2, 3):
>      setattr(X, 'f{}' % i, i)
>
> 2. generate code inline
>
> class Y: pass
>      # generate constants in local (class-)namespace
>      for i in (1, 2, 3):
>          locals()['f{}' % i] = i

Mutating class locals() currently works in CPython, but is explicitly 
not guaranteed to work by the language definition.

> In both cases, variables (loop variable 'i') are leaked into the
> surrounding namespace, which is kind-of ugly. The second approach also
> seems a bit hackish and I can't use the class-in-definition there, which
> is limiting when you want to attach e.g. constants of type X to X.
>
>
>>> Also PEP 3115 "Metaclasses in Python 3000"[2] seems to
>>> consider postprocessing of a class definition as better handled by a
>>> class decorator, which is something I haven't looked at yet.
>>
>> Generally, class decorators are less brain-melting than metaclasses.
>
> Alas, they also need to be defined before the class, messing with the
> mentioned order of declaration. They can be used to call a class
> function though which then does the necessary postprocessing...
>
> 3. post-process the class triggered with decorator
>
> def postprocess_class(cls):
>      """invoke postprocess() on the decorated object"""
>      cls.postprocess()
>      del cls.postprocess
>      return cls
>
> @postprocess_class
> class Z:
>      @classfunction
>      def postprocess(cls):
>          # attach constants to class
>          for i in (1, 2, 3):
>              setattr(cls, 'f{}' % i, i)
>
>
> I guess I'll stay with variant 1 for now, since it requires the least
> amount of code and the least amount of questions from other developers
> here.