It looks like PyMem_RESIZE() would be a truer equivalent than PyMem_Calloc(), since PyMem_MALLOC() does not initialize the memory. I would be happy with changing to that if you want.

PyMem_Malloc() has been limited to PY_SSIZE_T_MAX since Issue 2620, although the documentation <https://docs.python.org/3.5/c-api/memory.html#c.PyMem_Malloc> only mentions “size_t”. There is no match for “ssize_t” etc anywhere on that page.