I was kinda guessing that modifying the slot list wasn't a good idea. ;)

My current use case is that I implement the "create" slot because it makes it very easy to intercept the spec and its configuration. It is not passed into "exec" as such, but I need it to initialise the module namespace with "__file__", "__path__", etc.

There is also still the idea of defining our own module type in Cython in order to have a place where we can keep C level module globals, and also to support module properties. PEP 549 will not be available in older Python versions, even if it gets accepted.

Having to choose between main-exec support and these two features seems wrong.