For this given python function :

file name demo.py

def return_tuple():
    return 1, 2, 3

To retrieve the values in ObjectScript, you can use the following code, basically use dunder methods to access the values.

set return = ##class(%SYS.Python).Import("demo")."return_tuple"()
write return."__len__"() // 3
write return."__getitem__"(0) // 1
write return."__getitem__"(1) // 2
write return."__getitem__"(2) // 3