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[Python-Dev] PEP 451 update

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On Sat, Oct 26, 2013 at 11:03 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:> I don't think we can postpone it to later, as we need to be clear on:>> 1. Does reload use __name__ or __spec__.name when both are available?> 2. Does __name__ get restored to its original value if reloading via> __spec__.name?> 3. Do other import related attributes get restored to their original values?> 4. Does create_module get called if the loader has an exec_module method?> 5. Does the loader have access to the previous spec when reloading a module?>> My answers to these questions (note that my answer to 2 differs from> what I had in my initial sketch):>> 1. __spec__.name> 2. Yes, __name__ is updated to match __spec__.name, *expect* if it is> currently "__main__"> 3. Yes, other import related attributes are restored to their baseline values> 4. No, create_module is never called when reloadingI agree on all of these.  I'm adding a "How reloading will work"section to the PEP.> 5. Currently no, but I'm thinking that may be worth changing (more on> that below)>> The reload() implementation in my message is actually based on the> current importlib.reload implementation. The only PEP 451 related> changes were:>> - using __spec__.name (if available) instead of __name__> - checking all parent modules exist rather than just the immediate parent module> - calling import.find_spec() rather than using the __loader__ attribute directly> - being explicit that __name__ is left at the value it had prior to the reload> - handling the namespace package, exec_module and no exec_module cases>> I also added an explicit check that the module was re-used properly,> but I agree *that* change is out of scope for the PEP and should be> considered as a separate question.>> Now, regarding the signature of exec_module(): I'm back to believing> that loaders should receive a clear indication that a reload is taking> place. Legacy loaders have to figure that out for themselves (by> seeing that the module already exists in sys.modules), but we can do> better for the new API by making the exec_module signature look like:>>     def exec_module(self, module, previous_spec=None):>         # module is as per the current PEP 451 text>         # previous_spec would be set *only* in the reload() case>         # loaders that don't care still need to accept it, but can> just ignore it>         ...I'd rather give loaders another optional method:supports_reload(name).  Complicating the spec methods signatures (tosupport passing the old spec through) just for the sake of reloadseems less than optimal.  I don't see any benefit to exposing the oldspec to loader.exec_module().>> So, with those revisions, the reload() semantics would be defined as:>>     def reload(module):>         # Get the name to reload from the spec if it is available,>         # otherwise use __name__ directly>         current_spec = getattr(module, "__spec__")>         current_name = module.__name__>         name_to_reload = current_name if current_spec is None else> current_spec.name>>         # Check this module is properly imported before trying to reload it>         loaded_module = sys.modules.get(name_to_reload)>         if loaded_module is not module:>             msg = "module {} not in sys.modules">             raise ImportError(msg.format(name_to_reload), name=name_to_reload)>         parent_name = name_to_reload.rpartition('.')[0]>         while parent_name:>             if parent_name not in sys.modules:>                 msg = "parent {!r} not in sys.modules">                 raise ImportError(msg.format(parent_name), name=parent_name)>             parent_name = parent_name.rpartition('.')[0]>>         # Check for a modified spec (e.g. if the import hooks have changed)>         updated_spec = importlib.find_spec(name_to_reload)>>         # The import-related module attributes get updated here>         _init_module_attrs(loaded_module, updated_spec)>         if current_name == "__main__":>             loaded_module.__name__ = "__main__">>         # Now re-execute the module>         loader = updated_spec.loader>         if loader is None:>             # Namespace package, already reinitialised above>             return loaded_module>         elif hasattr(loader, 'exec_module'):>             loader.exec_module(module, current_spec)>         else:>             loader.load_module(name_to_reload)>>         # Allow for the module to be replaced in sys.modules>         # (even though that likely won't work very well)>         return sys.modules[name_to_reload]This is pretty much the same thing I've implemented. :)-eric

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