def a(x): def b(y): # input y when print return x + y # y is not undefine at this moment until print, Closure on this line return b # b is just an address, undefined data type add10 = a(10) add20 = a(20) print(add10(5), add20(5)) print(a(10)(5),a(20)(5)) # same as line 7, so it is partial # above: Let function become a constant