The PyObject_CallFunction() function has a special case if the format string is "O". See my email thread on python-dev:
https://mail.python.org/pipermail/python-dev/2016-December/146919.html

Serhiy wrote: "It is documented for Py_BuildValue(), and the documentation of PyObject_CallFunction() refers to Py_BuildValue()." which is true, but I would prefer to be more explicit to avoid bad surprises (see issues #21209 and #28920).

Attached patch modifies PyObject_CallFunction() doc to mention the "O" format special case in the .rst doc and the .h comment.

The documentation of PyObject_CallMethod() and _PyObject_CallMethodId() should also be modified, but I would prefer to wait for a first review on the added text before modifying all functions.

I proposed to only modify Python 3.7 because Include/abstract.h was a in a bad shape: I rewrote this file to make it easier to maintain, but only in Python 3.7, see issue #28838.