traceback.clear_frames():
"Clears the local variables of all the stack frames in a traceback tb by calling the clear() method of each frame object."
https://docs.python.org/dev/library/traceback.html#traceback.clear_frames

That's wrong: it only calls frame.clear() on the top frame of each traceback object.

Output of attached clear_frames_bug.py script:
---
locals:
frame locals {'z': 3}
frame locals {'y': 2}
frame locals {'x': 1}

clear_frames()

locals:
frame locals {}
frame locals {'y': 2}
frame locals {'x': 1}

---

As you can see, only the locals of the func3() frame, where the exception was raised, is clear. Locals of other frames are not cleared.

Attached PR fixes the issue.

Output with the fix:
---
locals:
frame locals {'z': 3}
frame locals {'y': 2}
frame locals {'x': 1}

clear_frames()

locals:
frame locals {}
frame locals {}
frame locals {}

---

Would it be ok to backport such change to Python 3.6? It's a bugfix, no?

traceback.clear_frames() was added by bpo-1565525: commit 173a157e725579eec1f28f8c9d53d6761ba6c79f.

See bpo-31323 where I wanted to use traceback.clear_frames() to manually break a reference cycle.