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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Behavior change for opcode trace after PEP 669","articleBody":"Before PEP 669, the opcode trace is togglable after trace function being set. So something like\r\n\r\n```python\r\nimport sys\r\n\r\ndef opcode_trace_func(frame, event, arg):\r\n    print(frame, event)\r\n    return opcode_trace_func\r\n\r\nsys.settrace(opcode_trace_func)\r\nsys._getframe().f_trace = opcode_trace_func\r\nsys._getframe().f_trace_opcodes = True\r\n\r\ndef f():\r\n    a = [1, 2, 3]\r\n    return a[0]\r\n\r\nf()\r\n```\r\n\r\nwould work.\r\n\r\nAfter PEP 669, the opcode event is set on `sys.settrace()` so it's impossible to enable it after enabling the trace.\r\n\r\n```python\r\nsys._getframe().f_trace_opcodes = True\r\nsys.settrace(opcode_trace_func)\r\nsys._getframe().f_trace = opcode_trace_func\r\n```\r\n\r\nkind of works but it's super intuitive.\r\n\r\nFor the current implementation, if we enabled opcode before we do `sys.settrace()`, it would be \"enabled\" forever. Meaning the event will be always on, the callback will be always triggered, but `frame.f_trace_opcode` will be checked in the callback function. Can we always have opcode callback on? It would be a performance hit(extra call for each opcode), but that happens if we want any opcode event at all, and I think that's basically how the old tracing system does it.\r\n\r\nAnother option would be making `f_trace_opcode` a descriptor and toggle(actually, probable just enable) opcode events there.\r\n\r\nIf we don't want to do that at all and just want to break the backward-compatibility because no one is doing insturction trace anyway (I only realized this when syncing https://github.com/python/cpython/pull/103050/ to the lastest change), we should probably document it - make sure the current frame's `f_trace_opcode` is set before calling `sys.settrace()` if you want opcode events.\r\n\r\nBTW we would've caught this if we already had the instruction level debugging :)\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-109472\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/gaogaotiantian","@type":"Person","name":"gaogaotiantian"},"datePublished":"2023-04-18T18:09:12.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":6},"url":"https://github.com/103615/cpython/issues/103615"}