Improvements Related to Namespace Package Portions for PathEntryFinders

tl;dr there are some oddities we should consider addressing with namespace package portions coming from path entry finders. It probably isn't worth bothering though (except for the docs fixes).

PEP 420 added (implicit) namespace packages. At that time, and since then, namespace packages (and portions) have gotten special-casing throughout the import machinery. Generally, we treat them like normal modules (or specs) even though certain characteristics don't apply. This isn't a problem generally because namespace packages aren't that common (and because, I expect, only builtin importers ever deal with them). However, there are a few things that it might be worth addressing.

PEP 451 introduced ModuleSpec, etc. PathEntryFinder.find_spec() as the replacement for PathEntryFinder.find_loader(), with some notable differences:

• find_loader() returns (loader, portion)
  • the interpretation of this value is clearly documented
  • the finder can't provide any other possible information
  • if the returned loader is not None then the portion is ignored (not even a warning is set)
• find_spec() returns a ModuleSpec with loader set to None and submodule_search_locations set to the portion
  • the interpretation of this value for portions is not clearly documented anywhere
  • the path entry finder may set addition info on the spec (e.g. loader_state) but it is all ignored (not even a warning if set)

In both cases, the language reference is not clear about how to identify namespace package portions from a path entry finder.

What to Do?

• update the PathEntryFinder.find_spec() docs entry
• (a weak maybe) add the details to the language reference
• (maybe) be more explicit when returning a namespace package portion
• (maybe) strictly disallow (or warn) for extraneous information tied to a portion

explicitly identify namespace package portions

1. do nothing
2. use a dedicated singleton instead of None
3. use a dedicated ModuleSpec subclass (e.g. NamespacePackagePortionSpec)
4. use a dedicated ModuleSpec-compatible class (e.g. NamespacePackagePortionSpec)

explicitly deal with unexpectedly set data

(This is basically just if spec.origin is set.)

1. do nothing
2. emit a warning
3. fail

explicitly deal with spec.loader_state

1. do nothing
2. emit a warning
3. fail
4. preserve the info on the spec of the combined namespace package

Comienza con la entrada de documentación de PathEntryFinder.find_spec() y la sección language-reference namespace-packages enlazada en el issue. Lee el contexto de PEP 420 y PEP 451, y después revisa las alternativas aún no resueltas para identificar las partes y gestionar datos adicionales de Spec. Se considera terminado cuando el comportamiento elegido por el proyecto está documentado con claridad y cualquier cambio de comportamiento relacionado se ha acordado antes de la implementación.