[Zope-dev] unregistering persistent utilities

[David Glick]

Is there a recommended way or example code somewhere for removing a  
utility registration from a persistent component registry, when that  
utility's module no longer exists?

The use case is in Plone, which has a local  
zope.app.cache.ram.IRAMCache utility.  Now that Hanno has factored out  
the RAM cache into zope.ramcache, we'd like the persistent utility to  
reference zope.ramcache, so that we can remove our zope.app.cache  
dependency.

I've been trying to make this work using a temporary module alias  
(code below), but something strange is going on.  In particular,  
unghosting the persistent utilities registry is affecting things  
somehow...before that happens, doing a repr of IRAMCache gives:
(Pdb) zope.app.cache.interfaces.ram.IRAMCache
<InterfaceClass zope.ramcache.interfaces.ram.IRAMCache>
...(what I'd expect, I guess, given my module alias)...but after the  
registry is unghosted, it gives:
(Pdb) zope.app.cache.interfaces.ram.IRAMCache
<InterfaceClass zope.app.cache.interfaces.ram.IRAMCache>
...(which is what I'm actually hoping for, in order to unregister the  
old utility)...but then I also get:
(Pdb) zope.ramcache.interfaces.ram.IRAMCache
<InterfaceClass zope.app.cache.interfaces.ram.IRAMCache>
...(e.g., now accessing it via the new module gives a class that  
thinks its in the old module)

Sorry for the rather muddled description, but I haven't been able to  
really understand what's going on there yet.

So what am I missing or doing wrong?  Is there a better way to go  
about this?


def relocateRamCacheUtility(portal):

     # temporary module alias
     try:
         import zope.app.cache
         old_zope_app_cache = zope.app.cache
     except ImportError:
         old_zope_app_cache = None
     import zope.ramcache
     zope.app.cache = sys.modules['zope.app.cache'] = zope.ramcache

     sm = getSiteManager(context=portal)
     # make sure the persistent utilities registry is unghosted before  
we
     # reference the old interface name, as it seems to make a  
difference in
     # whether it can be unregistered (WTF?)
     sm.utilities.changed

     component = sm.getUtility(zope.app.cache.interfaces.ram.IRAMCache)
     sm.unregisterUtility(component=component,
         provided=zope.app.cache.interfaces.ram.IRAMCache)
     import zope.ramcache
     import pdb; pdb.set_trace( )
     sm.registerUtility(factory=zope.ramcache.ram.RAMCache,
         provided=zope.ramcache.interfaces.ram.IRAMCache)

     logger.info('Updated the import location of the local IRAMCache  
utility.')

     # reset the module alias
     zope.app.cache = sys.modules['zope.app.cache'] = old_zope_app_cache

thanks,

David Glick
Web Developer
ONE/Northwest

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