Worth noting is that there is an existing loop-breaking mechanism, but only for the newest exception being raised. In particular, option (4) is actually the current behavior if the the most recent exception participates in a cycle:
Python 3.9.0b1 >>> A, B, C, D, E = map(Exception, "ABCDE") >>> A.__context__ = B >>> B.__context__ = C >>> C.__context__ = D >>> D.__context__ = E >>> try: ... raise A ... except Exception: ... raise C ... Exception: B During handling of the above exception, another exception occurred: Traceback (most recent call last): File "<stdin>", line 2, in <module> Exception: A During handling of the above exception, another exception occurred: Traceback (most recent call last): File "<stdin>", line 4, in <module> Exception: C This cycle-breaking is not due to any magic in the ``PyException_SetContext()``, which is currently a basic one-liner, but instead comes from ``_PyErr_SetObject`` in errors.c, which has something along the lines of: def _PyErr_SetObject(new_exc): top = existing_topmost_exc() if top is None: # no context set_top_exception(new_exc) return # convert new_exc class to instance if applicable. ... if top is new_exc: # already on top return e = top while True: context = e.__context__ if context is None: # no loop break if context is new_exc: # unlink the existing exception e.__context__ = None break e = context new_exc.__context__ = top set_top_exception(new_exc) The only trouble comes about when there is a "rho-shaped" linked list, in which we have a cycle not involving the new exception being raised. For instance, Raising A on top of (B -> C -> D -> C -> D -> C -> ...) results in an infinite loop. Two possible fixes would be to either (I) use a magical ``__context__`` setter to ensure that there is never a rho-shaped sequence, or (II) allow arbitrary ``__context__`` graphs and then correctly handle rho-shaped sequences in ``_PyErr_SetObject`` (i.e. at raise-time). Fix type (I) could result in surprising things like: >>> A = Exception() >>> A.__context__ = A >>> A.__context__ is None True so I propose fix type (II). This PR is such a fix: https://github.com/python/cpython/pull/20539 It basically extends the existing behavior (4) to the rho-shaped case. It also prevents the cycle-detecting logic from sitting in two places (both _PyErr_SetObject and PyException_SetContext) and does not make any visible functionality more magical. The only Python-visible change should be that the infinite loop is no longer possible. _______________________________________________ Python-Dev mailing list -- [email protected] To unsubscribe send an email to [email protected] https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/[email protected]/message/R5J5JVUJX3V4DBKVLUI2SUBRD3TRF6PV/ Code of Conduct: http://python.org/psf/codeofconduct/