[Python-Dev] advice needed: best approach to enabling "metamodules"?

[Nick Coghlan]

On 29 November 2014 at 21:32, Antoine Pitrou <solipsis at pitrou.net> wrote:
> On Sat, 29 Nov 2014 01:59:06 +0000
> Nathaniel Smith <njs at pobox.com> wrote:
>>
>> Option 1: Make it possible to change the type of a module object
>> in-place, so that we can write something like
>>
>>    sys.modules[__name__].__class__ = MyModuleSubclass
>>
>> Option 1 downside: The invariants required to make __class__
>> assignment safe are complicated, and only implemented for
>> heap-allocated type objects. PyModule_Type is not heap-allocated, so
>> making this work would require lots of delicate surgery to
>> typeobject.c. I'd rather not go down that rabbit-hole.
>
> Option 1b: have __class__ assignment delegate to a tp_classassign slot
> on the old class, so that typeobject.c doesn't have to be cluttered with
> many special cases.

Aye, being able to hook class switching could be potentially useful
(including the ability to just disallow it entirely if you really
wanted to do that).

>> Option 3: Make it legal to assign to the __dict__ attribute of a
>> module object, so that we can write something like
>>
>>    new_module = MyModuleSubclass(...)
>>    new_module.__dict__ = sys.modules[__name__].__dict__
>>    sys.modules[__name__].__dict__ = {}     # ***
>>    sys.modules[__name__] = new_module
>>
> [...]
>>
>> Option 4: Add a new function sys.swap_module_internals, which takes
>> two module objects and swaps their __dict__ and other attributes. By
>> making the operation a swap instead of an assignment, we avoid the
>> lifecycle pitfalls from Option 3. By making it a builtin, we can make
>> sure it always handles all the module fields that matter, not just
>> __dict__. Usage:
>
> How do these two options interact with the fact that module functions
> store their globals dict, not the module itself?

Right, that's the part I consider the most challenging with
metamodules - the fact that there's a longstanding assumption that a
module is just a "dictionary with some metadata", so the interpreter
is inclined to treat them that way.

Cheers,
Nick.

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