class definition error in python

Question

What is wrong with defining a class constructor like this:

I am trying to construct two different objects based on whether input d stays None or gets value assigned.

class MSMeshFace(object):

    def __init__(self, a= None, b= None, c= None, d= None):
            self.a = a
            self.b = b
            self.c = c
            self.d = d
            if self.d == None:
                    triangleFace = MSMeshFace(self.a, self.b, self.c)
                    self.count = 3
                    return triangleFace
            else:
                    quadFace = MSMeshFace(self.a, self.b, self.c, self.d)
                    self.count = 4
                    return quadFace

Answer 1

The constructor (well, the initializer, really) is not supposed to return anything, it's supposed to initialize a newly created instance. What you want is:

class MSMeshFace(object):
    def __init__(self, a=None, b=None, c=None, d=None):
        self.a = a
        self.b = b
        self.c = c
        self.d = d
        self.count = 3 if self.d is None else 4

Answer 2

If you are trying to return an object of different types based on arguments, make a new function like:

def make_face(*args):
    if len(args) == 3:  # triangle face
        return TriMeshFace(*args)
    else:  # quad face
        return QuadMeshFace(*args)

You can't (normally) change type in a constructor (you may be able to in __new__ , though, but you don't need that for this). If you want to add functions to MSMeshface (as you suggest in the comments), define a base class containing those functions such as:

class MeshBase:
    def addData(self, data): pass
    def ToDSMeshFace(self): pass

class TriMeshFace(MeshBase): pass

class QuadMeshFace(MeshBase): pass

Answer 3

Your intention seems to be as follows:

class MSMeshFace(object):
    def __init__(self, a= None, b= None, c= None, d= None):
        self.a = a
        self.b = b
        self.c = c
        self.d = d
        self.count = 3 if self.d is None else 4

The constructed object will be returned automatically. The object already exists, and is pointed by self . You can't influence the moment of creation of the object in the class's __init__ method, you can only initialize it.