Drilling down in a dict with "complex" objects

[Chris Rebert]

On Mon, May 24, 2010 at 9:34 PM, Six <john.d.perkins at gmail.com> wrote:
> I am trying to access an objects sub-object attributes. I can boil the
> code I am working with down to this problem section:
> (snip)
> class Pt:
>  x = None
>  y = None
>  def __init__(self, x, y):
>    self.x, self.y = x, y
>  pass
>
> class Pts:
>  curr_point = None
>  next_point = None
>  def __init__(self, n, m):
>    self.next_point = Pt(n, m)
>  def update(self, point):
>    self.curr_point = self.next_point
>    self.next_point = point
>
> class PtManage:
>  points = {}
>  def __init__(self):
>    pass
>
> point = Pts(3,5)
> pman = PtManage()
> pman.points["odds"] = point
> print dir(pman)
>
> print pman["odds"].next_point.x
>
> (snip)
>
> It's this last line that doesn't work. What am I doing wrong? Is this
> a failure of the design or am I missing something obvious? How do I
> get down and see that "Pt" classes x attribute within the PtManage
> dict?

I suggest you read the part of Python's tutorial concerning classes
(http://docs.python.org/tutorial/classes.html ). Note that "curr_point
= None" and similar at the class level *does not* declare an object
field, because Python does not have instance variable declarations.
Here is a fixed and normalized version of the classes in your example:

class Pt(object):
    def __init__(self, x, y):
        self.x, self.y = x, y

class Pts(object):
    def __init__(self, n, m):
        self.curr_point = None
        self.next_point = Pt(n, m)
    def update(self, point):
        self.curr_point = self.next_point
        self.next_point = point

class PtManage(object):
    def __init__(self):
        self.points = {}


As for why your last line fails:
> print pman["odds"].next_point.x
As Sean said, you're missing a ".points":
print pman.points["odds"].next_point.x

Also, is there any reason for PtManage over just using a `points`
dictionary directly?

Cheers,
Chris
--
http://blog.rebertia.com