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.

Did you check this: [possible answer](https://stackoverflow.com/questions/141545/how-to-overload-init-method-based-on-argument-type) — maverick6912, Jan 20 '20 at 18:58
Why don't you want to use `**kwargs` instead of list of arguments? — lampslave, Jan 20 '20 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, Jan 20 '20 at 19:04
@fireball.1 did you read this [answer](https://stackoverflow.com/a/141777/7664461) — maverick6912, Jan 20 '20 at 19:12
@maverick6912 yes but it doesn't help with multiple arguments and passing nothing or None for the 5 missing arguments — fireball.1, Jan 20 '20 at 19:18
You want positional only parameters/arguments? With different ordering in the call signature? — wwii, Jan 20 '20 at 19:35
If you want the omitted arguments to be `None`, why not just make their default values `None`? — kaya3, Jan 20 '20 at 19:48

score 1 · Accepted Answer · answered Jan 20 '20 at 19:44

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}

Using multiple constructors for a class with defined number of arguments

1 Answers1