IIRC, the reason for DummyMod is because we can't assign the __dict__ of a module directly. If user_ns and user_module aren't both passed in, we want to ensure we have only one namespace. If user_module.__dict__ is not user_ns, then things quickly get more complicated - I think this was giving us issues with pickling, in fact.

• If user_module is passed in, we use its __dict__ as user_ns.
• If neither is passed in, we create a new instance of ModuleType, and use its __dict__ as user_ns.
• If user_ns is passed in, we create a DummyMod and assign user_ns to its __dict__.

There's more discussion of the rationale in PRs #384 and #648.

@takluyver: popped out the 2.6 compatibility changes as requested. PR #4186 is now just moving DummyMod to the module's namespace. Sorry was a bit thrown off since README.rst still states 2.6 compatible. I do remember vaguely the "drop 2.6" discussion from the dev mailing list.

👍 Thanks Mike. We'll get the README updated.

Thanks @mmckerns . I think this looks sound, but I'll take a few minutes to test it after lunch.

This works fine in my manual tests - with those couple of small doc changes, I'll merge it.

updated as requested. nice working with you on this.

Merging :-)