Inheriting from Python list object(type?)

[irstas at gmail.com]

On May 23, 10:07 pm, Mangabasi <mangab... at gmail.com> wrote:
> This is the winner:
>
> class Point(list):
>     def __init__(self, x, y, z = 1):
>         super(Point, self).__init__([x, y, z])
>         self.x = x
>         self.y = y
>         self.z = z
>
>     def __getattr__(self, name):
>         if name == 'x':
>             return self[0]
>         elif name == 'y':
>             return self[1]
>         elif name == 'z':
>             return self[2]
>         else:
>             return self.__dict__[name]
>
>     def __setattr__(self, name, value):
>         if name == 'x':
>             self[0] = value
>         elif name == 'y':
>             self[1] = value
>         elif name == 'z':
>             self[2] = value
>         else:
>             self.__dict__[name] = value


Inheritation is an "is a" relation. Point, however, is not a list.
This adds some weird behaviour to Point:

p = Point(1,2,3)
p.append(4)
print p[3]

That makes no sense but your Point implementation allows it. This
is probably even more surprasing behaviour:

p = Point(1,2,3) + Point(4,5,6)
print p

One might expect the points to be added to Point(5,7,9), not
into a list containing [1,2,3,4,5,6].

I'd suggest you don't inherit Point from anything, and just add
an __iter__ member function that iterates through x,y and z. E.g.

def __iter__(self):
    yield self.x
    yield self.y
    yield self.z

Then you can convert a Point p to a list by doing list(p). Or to
a tuple with tuple(p). __array__ could also be implemented for
convenince with numpy (if I understood that correctly).