On 29 November 2016 at 20:54, Tomas Orsava <torsava@redhat.com> wrote:
> With a metapath hook, .missing.py files are probably overkill, and the hook
> can just look at one file (or a static compiled-in list) of
> ModuleNotFound/ImportError messages for all missing modules, as M.-A.
> Lemburg and others are suggesting. We'll just need to think about
> coordinating how the list is generated/updated: the current PEP implicitly
> allows other parties, besides Python and the distributors, to step in
> cleanly if they need to—needing to update a single list could lead to messy
> hacks.
What if, rather than using an explicitly file-based solution, this was
instead defined as a new protocol module, where the new metapath hook
imported a "__missing__" module and called a particular function in it
(e.g. "__missing__.module_not_found(modname)")?
You can answer this question the best, Nick, but would it be worth defining a _stdlib.py that acts as both a marker for where the stdlib is installed -- instead of os.py which is the current marker -- and which also stores metadata like an attribute called `missing` which is a dict that maps modules to ModuleNotFoundError messages? Although maybe this is too specific of a solution (or still too general and we use an e.g. missing.json off of sys.path which contains the same mapping).
Otherwise MAL touched on the solution I always had in the back of my head where we let people register a callback that gets passed the name of any module that wasn't found through sys.meta_path. We could either have the return value mean nothing and by default raise ModuleNotFoundError, have the return value be what to set the module to and raise an exception as expected, or have it be more error-specific and return an exception to raise (all of these options also ask whether a default callback doing what is normal is provided or if it's None by default and import continues to provide the default semantics). The perk of the callback is it removes the order sensitivity of any sys.meta_path or sys.path_hooks solution where people might be doing sys.meta_path.append(custom_finder) and thus won't necessarily trigger if a new hook for missing modules is put at the end by default.