50. Argument Packing and Unpacking

Argument packing, and unpacking, is common in Python and especially, in Kivy.

---

The print function is imported from __future__ module for Python 3 compatibility. A function calc1() has 3 arguments.

---

The function calc2() and calc3() are almost identical. Because of the * symbol, args will pack the arguments it receives into a tuple. In calc2(), we use the individual elements to define the local variables a, b, and c. In calc3(), we use unpacking of args to three local variables.

---

In the main code, the same arguments are used for calc1(), calc2(), and calc3(). The results are placed in val1, val2, and val3. We know they should be the same.

---

If we have a list, or tuple with the parameters, the * symbol may be used to unpack – that is, if we have a 3-length list, it becomes 3 variables. If we did not use a * symbol, Python would have complained about not finding 3 arguments.

# ex50.py

from __future__ import print_function

def calc1(a,b,c):
    return 2*a+3*b+4*c

def calc2(*args):
    a = args[0]
    b = args[1]
    c = args[2]
    return 2*a+3*b+4*c

def calc3(*args):
    a,b,c = args
    return 2*a+3*b+4*c

def on_touch_down(self,*args):
    touch,res = args
    print(touch)
    print(res)

    
if __name__=='__main__':
    val1 = calc1(1,2,3)
    print('val1 =',val1)
    val2 = calc2(1,2,3)
    print('val2 =',val2)
    val3 = calc3(1,2,3)
    print('val3 =',val3)
    in1 = 1,2,3
    val4 = calc1(*in1)
    print('val4 =',val4)
    on_touch_down(1,1)


    

---

The result shows that val1 to val4 are equal, with a value of 20. For parameters with names, we can also have dictionary packing and unpacking.

---

We can always use *args, in function definitions. For example, for on_touch_down we can use *args to convert the argument into a tuple. However, that tuple will only have one element. *args is usually used if you have many arguments, or even if you have a variable number of arguments.

---