I don't think this is an appropriate fix, since in most cases there is no need to prevent other Python threads running while inside RegSetValue. There are also other ways that a context switch may occur during the enumeration which will put the program in exactly the same state. (It isn't clear from the patch, but the two sections of changed code are from completely different functions.)

The correct fix should be at the user's application level, or in a higher-level module than _winreg is supposed to be.