Comment 3 for bug 1720354

It sounds like the policy library is not clear enough that it expects the thing passed to it to be a dictionary, or at least to fully act like one.

@Cyril, I think your pastebin from comment #1 is going in the right direction. Would it fill out the API of ImageTarget so that it is fully compliant with the Mapping abstract base class? https://docs.python.org/2/library/collections.html#collections.Mapping