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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Incomplete stack traces when throwing into a generator chain that ends in a custom generator","articleBody":"# Bug report\r\n\r\n### Bug description:\r\n\r\n[bpo-29590](https://bugs.python.org/issue29590) identified an issue where only the head of a chain of generators was included in stack-traces when processing a thrown-in exception. I think this issue still exists but only in the case where the head of a chain of generators is a custom generator.\r\n\r\nConsider the following example in which a custom generator's `throw()` method does not see its predecessor in the `yield from` chain (the `g()` generator), but the builtin in generator does:\r\n```python\r\n$ cat test.py\r\nimport sys\r\n\r\nclass UserGen:\r\n  def throw(self, *args):\r\n    print(\"Traceback from UserGen:\")\r\n    f = sys._getframe()\r\n    while f:\r\n      print(f)\r\n      f = f.f_back\r\n  def __iter__(self):\r\n    return self\r\n  def __next__(self):\r\n    return 42\r\n\r\ndef real_gen():\r\n  yield 43\r\n\r\ndef g(target):\r\n  yield from target\r\n\r\ngg = g(UserGen())\r\ngg.send(None)\r\ngg.throw(RuntimeError)\r\n\r\nprint()\r\n\r\ngg = g(real_gen())\r\ngg.send(None)\r\ngg.throw(RuntimeError)\r\n\r\n\r\n$ python3 test.py\r\nTraceback from UserGen:\r\n\u003cframe at 0x10273e230, file 'test.py', line 8, code throw\u003e\r\n\u003cframe at 0x10270d440, file 'test.py', line 26, code \u003cmodule\u003e\u003e\r\n\r\nTraceback (most recent call last):\r\n  File \"test.py\", line 32, in \u003cmodule\u003e\r\n    gg.throw(RuntimeError)\r\n  File \"test.py\", line 22, in g\r\n    yield from target\r\n  File \"test.py\", line 18, in real_gen\r\n    yield 43\r\nRuntimeError\r\n```\r\n\r\nThe fix for [bpo-29590](https://bugs.python.org/issue29590) is in #19896 and appears to specifically only link frames together for chains of builtin generators only. I *suspect* this was just an oversight rather than a deliberate choice though.\r\n\r\nI attach a proposed fix in a PR.\r\n\r\n### CPython versions tested on:\r\n\r\nCPython main branch\r\n\r\n### Operating systems tested on:\r\n\r\n_No response_\r\n\r\n\u003c!-- gh-linked-prs --\u003e\r\n### Linked PRs\r\n* gh-126092\r\n\u003c!-- /gh-linked-prs --\u003e\r\n","author":{"url":"https://github.com/jbower-fb","@type":"Person","name":"jbower-fb"},"datePublished":"2024-10-28T20:32:23.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/126091/cpython/issues/126091"}