A snippet from my user-init.scm:

(pyindexed-set! (py-dict (current-pymodule))
                (string->pystring "magick")
                (procedure->pycallable
                 (lambda (a b) (integer->pyint (+ (pyint->integer a) (pyint->integer b))))))
(write (pyeval "magick (10, 20)"))
(newline)
(pyexec "print (magick (40, 50))")

The output:

#<pyobject repr:30 0x3b96210>
90