__init__ style questions

[bearophileHUGS at lycos.com]

Will McGugan:
> 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).

If optimizations are less important, then don't use __slots__, it
simplifies OOP management of it.

I think that accepting a single iterable too makes the calling a bit
too much elastic, so it can produce silent problems. Something like
this may be better:

from itertools import imap

class Vector3D(object):
    def __init__(self, *args):
        len_args = len(args)
        if len_args == 3:
            self.x, self.y, self.z = imap(float, args)
        elif len_args == 0:
            self.x = self.y = self.z = 0
        else:
            raise TypeError("...")

If you don't like imap, you can change that code.
If you want to accept single parameter too then you can use something
like:

class Vector3D(object):
    def __init__(self, *args):
        len_args = len(args)
        if len_args == 3:
            self.x, self.y, self.z = imap(float, args)
        elif len_args == 1:
            self.x, self.y, self.z = imap(float, args[0])
        elif len_args == 0:
            self.x = self.y = self.z = 0
        else:
            raise TypeError("...")

If you don't like the explicit raising of an error you may use:

class Vector3D(object):
    def __init__(self, first=None, *other):
        if first:
            if other:
                self.x = float(first)
                self.y, self.z = imap(float, other)
            else:
                self.x, self.y, self.z = imap(float, first)
        else:
            self.x = self.y = self.z = 0

But this last code is less readable.

Bye,
bearophile