Mark Dickinson added the comment: Yes, PyLong_FromString is directly used by the implementation of int, and is also used in parsing of numeric integer literals in source: https://github.com/python/cpython/blob/cb41b2766de646435743b6af7dd152751b54e... So I agree that this is a documentation bug. There's also no mention of the support for underscores in the documentation. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue29751> _______________________________________