[Import-SIG] PEP 451 (ModuleSpec) round 3

[Nick Coghlan]

On 31 August 2013 00:57, Brett Cannon <brett at python.org> wrote:
>> So perhaps a better name might be "prepare_module" (by analogy to PEP
>> 3115), and have it accept a "reloading" parameter, which is an
>> existing module to be reused.
>
>
> Is this to replace create_module() or exec_module()?

It replaces create_module.

>> The signature would be something like:
>>
>>     def prepare_module(reloading=None):
>>         """Create a module object for execution. Returning None will
>> created a default module.

Oops, stuffed up the signature. First arg should be the module spec:

    def prepare_module(spec, reloading=None):
        ...

> I can't follow that sentence. =) What does returning None represent?

Returning None indicates that the *loader* defines a module creation
API, but the particular module being loaded doesn't take advantage of
it.

It's a feature I need for the new extension module loader API, where
the creation hook allows the extension module to build a completely
custom object (perhaps with additional state). You can request an
ordinary module just by not defining the creation hook, and only
defining the execution hook (which accepts an already created module).

By switching to a *preparation* hook, rather than creation, I think we
can make this play more nicely with reloading. In the reloading case,
the preparation hook would be responsible for checking that the
existing object was a suitable execution target.

>>         If *reloading* is set, specifies an existing sys.modules entry
>> that is being reloaded.
>
> As in the key into sys.modules?

No, as in the object itself. Technically it doesn't *have* to be in
sys.modules, and the loader really shouldn't care if it is or not.

>> Must return None or that
>>         specific object if reloading is supported.
>
>
> What's "that" supposed to represent?

s/that specific object/the passed in object/

>> Returning a
>> different module object or explicitly raising ImportError
>>         indicates that reloading is not supported. (Or perhaps define
>> a "ReloadError" subclass of ImportError?)
>>         """
>
>
> I'm really not following what this method is supposed to do. Is it simply
> mucking with sys.modules? Is it creating a module to use? If it's the latter
> then how does return None do anything? Are you saying returning None means
> "I didn't do anything special, do what you want"?

It replace create_module with something that can also serve as the
pre-check for the reloading case.

Cheers,
Nick.

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