[Python-3000] Interest in PEP for callbacks on module import

[Christian Heimes]

Nick Coghlan wrote:
> For example, to handle the commented out case above:
> 
>    @imp.imported('decimal')
>    def register(decimal):
>        Inexact.register(decimal.Decimal)

I like the syntax assuming that imp.imported(name) expects a method that
accepts the module object as argument.

> I think a PEP would be needed to decide whether to handle this in a 
> fashion similar to that of PJE's Importing toolkit [1] (i.e., using lazy 
> imports where the actual loading of the module code is deferred until 
> the first access to an attribute of the module), or else to add a new 
> mechanism directly to the interpreter code, where the registered 
> callbacks would be called as soon as the specified module was loaded.

I find a new mechanism easier to implement. Lazy or deferred imports
require frame hacks and you know how Guido thinks about sys._getframe(). :)

An implementation based on your suggestion is simple. Add a registry to
imp which is a simple dict that maps dotted module names to a list of
callables. Upon import the IMP module calls the callables after the
module is inserted into sys.modules. That's it. :]

Zope's deferredimport [1] package has a nice idea. It supports
deprecation warnings when a deprecated attribute is accessed the first time.

>>> import somemodule

No warning is raised

>>> somemodule.deprecated_feature

DeprecationWarning ...

The package makes use of stack inspection and a proxy package.

> Does anyone else think this is an issue worth pursuing? 

Yes, count me in. I like to help with the PEP and implementation. :)

Christian

[1] http://svn.zope.org/zope.deferredimport/trunk/src/zope/deferredimport/