If module A imports module B then module B imports module A, what happens is:

 - module A is loaded once
 - module B is loaded
 - module A is loaded a second time
 - the second copy of module A is stored in imports.a
 - module B is stored in imports.b
 - the first copy of module A is stored in imports.a

Other modules may be using either the first or second copy of module.A depending on when they were loaded, and in general things go to hell in very unpredictable ways.

On import start, the module object needs to be immediately stored in imports.a 
so that other modules see it is there.

On failure, ideally all assignments to imports.* done during the import would be rolled back. Rolling back just the assignment to imports.a could leave things in an inconsistent state, since other modules might reference the dictionary. However, I think getting that right is low priority... it's much less of a pitfall if an exception during a complicated import causes a
inconsistent state, then if a succesful import causes an inconsistent state.

I'll attach a test case.