https://github.com/python/cpython/commit/abb9a448dee3e18c69080231fbeba980bf0... commit: abb9a448dee3e18c69080231fbeba980bf048211 branch: master author: Brandt Bucher <brandtbucher@gmail.com> committer: GitHub <noreply@github.com> date: 2020-02-01T11:08:34Z summary: Update sum comment. (#18240) files: M Python/bltinmodule.c diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c index 4f833c1f46253..5818eb9e38f6a 100644 --- a/Python/bltinmodule.c +++ b/Python/bltinmodule.c @@ -2440,7 +2440,11 @@ builtin_sum_impl(PyObject *module, PyObject *iterable, PyObject *start) empty = [] sum([[x] for x in range(10)], empty) - would change the value of empty. */ + would change the value of empty. In fact, using + in-place addition rather that binary addition for + any of the steps introduces subtle behavior changes: + + https://bugs.python.org/issue18305 */ temp = PyNumber_Add(result, item); Py_DECREF(result); Py_DECREF(item);