I have a use case where I need to put additional restrictions on object 
creation, in particular I need to restrict the maximum depth of items 
inside of a container of a specific type. The ideal place to put such a 
restriction seems to be the isConstructionAllowed method on the FTI. 
Currently this method is not very extensible, which leads to complicated 
code in various FTI types.

I am considering to add an extension point here, something like this:

class ITypeConstructionFilter(Interface):
     def __init__(fti, container):
         """Adapt on the FTI of the object being created and the target 

     def allowed():
         """Check if construction is allowed."""

current checks such as the workflow check that was added in CMF 2.2, or 
the type constraint logic Plone has in ATContentTypes could be moved to 
such an adapter. The standard isConstructionAllowed method could then 
query all registered adapters to check if construction should be possible.

Does this sound sensible?


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