[Zope] Set _allowed_meta_types on folder instances without creating a new product?

[Jeff Kowalczyk]

> I told you how to define "_allowed_meta_types" on a per instance
> basis but did not check that "_allowed_meta_types" has the desired
effect.
> When your message above arrived, I tried to find a use
> of "_allowed_meta_types", but the Zope sources do not
> reference it. Where do you know about it?

I've been using it in mxmObjectManager derived product classes to limit
the Metatype list.

class PurchaseOrder(PropertyManager, mxmObjectManager.mxmObjectManager):
 "A PurchaseOrder"
  meta_type = 'Purchase Order'
 _allowed_meta_types = ('Invoice','Item','Location','Shipment',)

It works exactly as expected, but it would be very useful to apply this
to existing folders which are specialized in purpose, but require no
special class of their own.

So I went looking into OFS ObjectManager and the mxmObjectManager
source, to see what Zope's native mechanism for this was:

mxmObjectManager.py ...
    # A cleaner way to filter metatypes
    # __allowed_meta_types = ('DTML Document','DTML Method')

    def all_meta_types(self):
        """
        Returns what meta_types can be added to the objectmanager
        """
        if hasattr(self, '_allowed_meta_types'):
            result = []
            for metaType in Products.meta_types:
                if metaType['name'] in self._allowed_meta_types:
                    result.append(metaType)
            return result
        else:
            return Products.meta_types

And OFS/ObjectManager.py
    def all_meta_types(self, interfaces=None):
	...

    def filtered_meta_types(self, user=None):
        # Return a list of the types for which the user has
        # adequate permission to add that type of object.
        user=getSecurityManager().getUser()
        meta_types=[]
        if callable(self.all_meta_types):
            all=self.all_meta_types()
        else:
            all=self.all_meta_types
        for meta_type in all:
            if meta_type.has_key('permission'):
                if user.has_permission(meta_type['permission'],self):
                    meta_types.append(meta_type)
            else:
                meta_types.append(meta_type)
        return meta_types


I'm more confused than before. Setting __all_allowed_metatypes causes a
"has key" error on the folder.