[issue30744] Local variable assignment is broken when combined with threads + tracing + closures

[Nick Coghlan]

Nick Coghlan added the comment:

I updated the old "we should clarify the semantics" issue with a more concrete update proposal: https://bugs.python.org/issue17960#msg296880

Essentially nothing would change for module and class scopes, but the proposal for function scopes is that locals() be changed to return "frame.f_locals.copy()" rather than a direct reference to the original.

Nothing would change for tracing functions (since they already access frame.f_locals directly), but the current odd side-effect that setting a trace function has on the result of normal locals() calls would go away.

Folks that actually *wanted* the old behaviour would then need to do either "sys._getframe().f_locals" or "inspect.currentframe().f_locals".

However, none of that would help with *this* issue: resolving the bug here would still require either a modification that allowed PyFrame_LocalsToFast to only write back those values that had been rebound since the preceding call to PyFrame_FastToLocals (ma_version_tag could help with doing that efficiently), or else a decision to disallow write-backs to frame locals even from tracing functions in 3.7+.

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<http://bugs.python.org/issue30744>
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