__init__ style questions

[Gerard Flanagan]

Will McGugan wrote:

> I am writting a Vector3D class as a teaching aid (not for me, for
> others), and I find myself pondering over the __init__ function. I want
> it to be as easy to use as possible (speed is a secondary
> consideration).
>
> Heres the __init__ function I have at the moment.
>
> class Vector3D(object):
>
>     __slots__ = ('x', 'y', 'z')
>
>     def __init__(self, x_or_iter=None, y=None, z=None):
>
>         if x_or_iter is None:
>             self.x = self.y = self.z = 0
>         elif z is None:
>             it = iter(x_or_iter)
>             self.x = float(it.next())
>             self.y = float(it.next())
>             self.z = float(it.next())
>         else:
>             self.x = float(x_or_iter)
>             self.y = float(y)
>             self.z = float(z)
>
> A Vector3D can be constructed in 3 ways. If no parameters are given it
> assumes a default of (0, 0, 0). If one parameter is given it is assumed
> to be an iterable capable of giving 3 values. If 3 values are given
> they are assumed to be the initial x, y, z.
>

here's a slightly different approach:


class Vector3D(object):
    __slots__ = ('x', 'y', 'z')

    def __init__(self, X1=None, X2=None, X3=None):
        if X3 is not None:
            #assume 3 numbers
            self.x = X1
            self.y = X2
            self.z = X3
        else:
            X1 = X1 or (0,0,0)
            X2 = X2 or (0,0,0)
            self.x = X1[0] - X2[0]
            self.y = X1[1] - X2[1]
            self.z = X1[2] - X2[2]

    def __getitem__(self, index):
        return getattr(self,self.__slots__[index])

    def __str__(self):
        return '(%s, %s, %s)' % (self.x, self.y, self.z )

u = Vector3D()
print u
u = Vector3D(3,4,5)
print u
u, v = Vector3D( [1,2,3] ), Vector3D( (3,2,1) )
print u, v
w = Vector3D( u,v )
print w
w = Vector3D( u, (2,2,2))
print w

(0, 0, 0)
(3, 4, 5)
(1, 2, 3) (3, 2, 1)
(-2, 0, 2)
(-1, 0, 1)

Gerard