[Python-Dev] PEP 451 update (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Sun Oct 27 21:59:24 CET 2013

On 28 Oct 2013 02:37, "PJ Eby" <pje at telecommunity.com> wrote:

On Sun, Oct 27, 2013 at 1:03 AM, Nick Coghlan <ncoghlan at gmail.com> wrote: > Now, regarding the signature of execmodule(): 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 execmodule signature look like: > > def execmodule(self, module, previousspec=None): > # module is as per the current PEP 451 text > # previousspec would be set only in the reload() case > # loaders that don't care still need to accept it, but can > just ignore it Just to be clear, this means that a lazy import implementation that creates a module object without a spec in the first place will look like an initial import? Or will that crash importlib because of a missing spec attribute? That is, is reload()'s contract adding a new prerequisite for the object passed to it? (The specific use case is creating a ModuleType subclass instance for lazy importing upon attribute access. Pre-importlib, all that was needed was a working name attribute on the module.)

For custom loaders, that's part of the contract for create_module() (since you'll get an ordinary module otherwise), and so long as setting the special module attributes doesn't cause the module to be imported during the initial load operation, attribute access based lazy loading will work fine (and you don't even have to set name, since the import machinery will take care of that).

For module level lazy loading that injects a partially initialised module object into sys.modules rather than using a custom loader or setting a spec attribute, yes, the exec_module invocation on reloading would always look like a fresh load operation (aside from the fact that the custom instance would already be in sys.modules from the first load operation). It will still work, though (at least, it won't break any worse than such code does today, since injecting a replacement into sys.modules really isn't reload friendly in the first place).

Cheers, Nick. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20131028/f7773f0c/attachment.html>

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