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I disagree. All that is doing is locking in the current poor interface.
We do need to extend the C API for these uses, yes. But we need an API that addresses Cython's needs directly and at a higher level.
For example, Cython often wants to insert a frame into the call stack for debugging and introspection.
Rather than have Cython laboriously create a Python frame with fake code object, etc., it would make much more sense for us to offer an API to insert a "native" frame into the stack and update the line number.
E.g. something like:
int PushNativeFrame(const char *filename);
void PopNativeFrame(void);
int CurrentNativeFrame_SetLineNumber(int lineno);
We might even want to use such an API ourselves in some modules.