a clear method for grok.Container

zope.app.container.btree.BTreeContainer is not inheriting from ZODB BTrees directly. In order to access the real btree data container, we have to go through the data container attribute (_SampleContainer__data). The BTree data container is a pure C object, it doesn't provide/implement anything, therefore it's impossible to test it against an interface.

I have a code working, with tests, for the "clear" method. The problem is : we would rely purely on the zope.container implementation and it's not good. Even more, zope.container.btree doesn't provide an object implementing a Btrees.Interfaces interface. Maybe we should make zope.container at least implementing Btrees.Interfaces.ICollection ? If not, we could, in Grok, provide a clean implementation that does really provide what we are expecting : a real BTrees.IBTree.

I'm not sure whether we are really expecting a real BTrees.IBTree? Is
IBTree an interface?

If there are improvements to the container to be done we should
explore doing so in zope.container. The _SampleContainer__data stuff
is ugly, so that'd be one more item for the wishlist. I'd also be very
nice to have a container that allowed people to override its __init__
without having to do a super call, if this can be done without too
much magic.

yes, IBtree is an interface defines in BTrees.Interfaces.

Fixing the _SampleContainer__data would be great, though I'm unsure about its current status. It's still marked as being there for a backward compatibility.