The chaotic locals() and how we fixed it
summary
Python changes local-variable semantics with PEP 667: locals() returns a fresh snapshot dict so reads never implicitly mutate fast locals and repeated calls return new dicts, while frame.f_locals becomes a write-through proxy that immediately updates the underlying C-array locals so debuggers can exec and mutate reliably. The implementation replaces ad hoc fast-to-locals hacks that caused PDB commands like ll to revert edits and it lands in CPython, removing the surprising behaviors that broke real debugger workflows.
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chapters (13)
- 0:00Introduction
- 1:10First bug in CPython
- 3:21Python local scope history
- 5:50Fast locals in CPython
- 8:03Broken locals and fast locals
- 11:15Exec and locals behavior
- 14:26Debugger challenges
- 18:23Locals behavior needs fixing
- 19:45PEP 667 goals
- 20:26PEP 667 semantics
- 21:11Frame.f locals proxy
- 23:21Debugger improvements
- 24:50Q&A with Mark Shannon
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