curses.filter forces the top most window to 1 line tall and preserves
existing tty content. It must be called *before* curses.initscr in order
to acheive this.

The python documentation states that curses.filter must be called before
initscr but the implementation prevents this. It uses the
NoArgNoReturnVoidFunction macro defined in Include/py_curses.h. This
macro in turn invokes the PyCursesInitialised macro which forces an
error if initscr has not been called.

curses.filter needs an explicit definition to avoid this:

Replacing "NoArgNoReturnVoidFunction(filter)" in Modules/_cursesmodule.c
with

static PyObject *PyCurses_filter (PyObject *self)
{
  /* MUST NOT BE PyCursesInitialised */
  filter(); \
  Py_INCREF(Py_None);
  return Py_None;
}

Would allow curses.filter to be called as documented. But really should
get a check for "!PyCursesInitialised".

You're right. I fixed this in r60322, r60323 (2.5).