It's interesting that we no longer use the default value of the "PythonPath" key itself. The old implementation in C used to include it when the skipcore argument was false, which was the case if calculate->home was NULL in the old calculate_module_search_path(). In other words, the default path from the registry was used as a fallback if the home landmark wasn't able to be found to set the prefix path.

@eryksun This will have been my oversight while doing the translation into Python. That said, I don't like that particular behaviour (any Python we install will locate its home already; another copy shouldn't rely on our install for a stdlib), so I'm happy to leave it out until someone shows a scenario that wouldn't be better off resolved in another way.