I think test class must bepublic.

This method violates single responsibility principle.

First,Create should never return an existing module. Instead, the caller should be able to check if module already exists by name by calling a separate function.

Second, I am uncertain about setting up__builtins__ by default. It would be great to know why Python does not do it before discarding that potentially good rationale. Users can always manually donewModule.Dict.Set("__builtins__", GetBuiltIns()). If this is a common scenario in your product, just make a factory method combining the two.

Third, I don't think adding new module tosys.modules right away is a good idea. Especially, overwriting the existing value. This might cause hard to debug surprise issues.

Second, I am uncertain about setting up__builtins__ by default. It would be great to know why Python does not do it before discarding that potentially good rationale. Users can always manually donewModule.Dict.Set("__builtins__", GetBuiltIns()). If this is a common scenario in your product, just make a factory method combining the two.

I didn't see a public method that would allow access to the __builtins__. There is an internal method, onRuntime, but not a public method that could be used outside.

The default value forfilename should benull and when it is null__file__ should simply not be set.