How to detect that a function argument is the default one

[ast]

Hello


Here is what I would like to implement

class Circle:

    def __init__(center=(0,0), radius=10, mass=1)):
        self.center = center
        self.radius = radius

        if  "mass is the default one":       <-----
            self.mass = radius**2
        else:
            self.mass = mass

I cant just test if mass == 1: because if I call
C1 = Circle((0,0), 7, 1)
then in that case I want: self.mass = 1

If i call  C2 = Circle((6,8), 8) 
then in that case I want: self.mass = radius**2

I have the idea to write:

def __init__(center=(0,0), radius=10, mass=None)):

    if  mass == None: 
        self.mass = radius**2
    else:
        self.mass = mass

but maybe Python provides something clever.

Thx