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Thanks for the details, Anatoly. I am surprised by the behavior you've described. It may be that functions don't use fast locals when tracing is turned on. I'll have to check. If that's the case, I think the documentation for locals() should be improved. Either way, I agree that inconsistent (and undocumented) behavior is a headache. However, changing the type of the object returned by locals() is going to take a lot more justification. A doc update should be sufficient and a much easier sell.
Here are some further questions:
1. the dynamic update you described happens in function bodies (i.e. stack frames)?
2. does that behavior happen when you are not using a tracing function?
Also, if you have a minute, throw up a patch that reproduces the behavior you're talking about. That will help get this resolved faster.
Keep in mind that locals() always returns a normal dict. Just like any other dict, other code that has a reference to that "locals" dict can interact with it, which is what you have described. In this case, the interpreter is doing so. The only case where I find that surprising is where a stack frame is using fast locals, which is what function calls normally do.