Using multiple constructors for a class with defined number of arguments

Question

class Atom:
    def __init__(self,x,y,z,element,charge,notused,fx,fy,fz):
        self.x=float(x)
        self.y=float(y)
        self.z=float(z)
        self.element=str(element)
        self.charge=float(charge)
        self.notused=int(float(notused))
        self.fx=float(fx)
        self.fy=float(fy)
        self.fz=float(fz)

    @classmethod
    def from_lammps(self,element,x,y,z):
        self.x=float(x)
        self.y=float(y)
        self.z=float(z)
        self.element=str(element)

I am trying to implement an alternate constructor for class Atom. However, I am not sure how what to do differently in from_lammps() to get an object with the given arguments only.

Why don't you want to use **kwargs instead of list of arguments? — lampslave
– lampslave, Commented Jan 20, 2020 at 19:00
@maverick6912 that seems to be calling the init method again. Can't associate None to the omitted arguments with the provided answer — fireball.1
– fireball.1, Commented Jan 20, 2020 at 19:04
You want positional only parameters/arguments? With different ordering in the call signature? — wwii
– wwii, Commented Jan 20, 2020 at 19:35

martineau · Accepted Answer · 2020-01-20 19:44:18Z

You could implement an alternative constructor with only the given arguments like this. Note that that the first argument that's passed to a classmethod is the class being instantiated.

class Atom:
    def __init__(self, x, y, z, element, charge, notused, fx, fy, fz):
        self.x = float(x)
        self.y = float(y)
        self.z = float(z)
        self.element = str(element)
        self.charge = float(charge)
        self.notused = int(float(notused))
        self.fx = float(fx)
        self.fy = float(fy)
        self.fz = float(fz)

    @classmethod
    def from_lammps(cls, element, x, y, z):
        obj = object.__new__(cls)
        obj.x = float(x)
        obj.y = float(y)
        obj.z = float(z)
        obj.element = str(element)
        return obj


from pprint import pprint, pformat

atom1 = Atom(1, 2, 3, 'zinc', '3.1415', 42, '4', '5', '6')
print(f'vars(atom1) =\n{pformat(vars(atom1))}')
print()
atom2 = Atom.from_lammps('silver', '6', '7', '8')
print(f'vars(atom2) =\n{pformat(vars(atom2))}')

Output:

vars(atom1) =
{'charge': 3.1415,
 'element': 'zinc',
 'fx': 4.0,
 'fy': 5.0,
 'fz': 6.0,
 'notused': 42,
 'x': 1.0,
 'y': 2.0,
 'z': 3.0}

vars(atom2) =
{'element': 'silver', 'x': 6.0, 'y': 7.0, 'z': 8.0}

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