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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Incorrect handling of negative `start` values on `PyUnicodeErrorObject`","articleBody":"# Bug report\r\n\r\n### Bug description:\r\n\r\nFound when implementing #123343. We have:\r\n\r\n```C\r\nint\r\nPyUnicodeEncodeError_GetStart(PyObject *exc, Py_ssize_t *start)\r\n{\r\n    Py_ssize_t size;\r\n    PyObject *obj = get_unicode(((PyUnicodeErrorObject *)exc)-\u003eobject,\r\n                                \"object\");\r\n    if (!obj)\r\n        return -1;\r\n    *start = ((PyUnicodeErrorObject *)exc)-\u003estart;\r\n    size = PyUnicode_GET_LENGTH(obj);\r\n    if (*start\u003c0)\r\n        *start = 0; /*XXX check for values \u003c0*/\r\n    if (*start\u003e=size)\r\n        *start = size-1;\r\n    Py_DECREF(obj);\r\n    return 0;\r\n}\r\n```\r\n\r\nThe line `*start = size-1` might set `start` to `-1` when `start = 0`, in which case this leads to assertion failures when the index is used normally.\r\n\r\n\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\nLinux\r\n\r\n\u003c!-- gh-linked-prs --\u003e\r\n### Linked PRs\r\n* gh-124935\r\n* gh-125098\r\n* gh-125099\r\n* gh-123380\r\n* gh-127739\r\n\u003c!-- /gh-linked-prs --\u003e\r\n","author":{"url":"https://github.com/picnixz","@type":"Person","name":"picnixz"},"datePublished":"2024-08-27T07:05:10.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":20},"url":"https://github.com/123378/cpython/issues/123378"}