This is an interesting find - thanks.

Adding Larry as 3.5 release manager to the cc here, as I think the right fix actually involves an API adjustment inside _imp.

One of the consequences of PEP 489 (multi-phase initialisation) was that builtin imports and extension module imports now share a lot more code, and that includes the fact that while we have both _imp.create_builtin and _imp.create_dynamic, they currently share a module level exec implementation in _imp.exec_dynamic.

However, the definition and implementation of _imp.exec_dynamic is still behind the same preprocessor guard as _imp.create_dynamic, hence the error reported here.

As far as a fix goes, rather than defining _imp.exec_dynamic unconditionally, my inclination is to say that merging the Python level APIs in addition to the underyling C level implementation represents a design mistake when we implemented the PEP, and there should be a separate _imp.exec_builtin function.

The fact that _imp.exec_dynamic just called _imp.exec_builtin internally would then be a pure CPython implementation detail, rather than something importlib specifically depended on.