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Re: Newbie: question regarding references and class relationships

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From Peter Otten <__peter__@web.de>
Subject Re: Newbie: question regarding references and class relationships
Date Mon, 10 Jun 2013 15:49:27 +0200
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Xref csiph.com comp.lang.python:47557

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Rui Maciel wrote:

> Let:
> - class Point be a data type which is used to define points in space
> - class Line be a data type which possesses an aggregate relationship with
> objects of type Point
> - class Model be a container class which stores collections of Point and
> Line objects
> 
> 
> Essentially, a Model object stores lists of Point objects and Line
> objects, and Line objects include references to Point objects which
> represent the starting and ending point of a line.
> 
> To reflect this class relationship, I've defined the following classes:
> 
> <code>
> class Point:

Don't add 

>         position = []

to your code. That's not a declaration, but a class attribute and in the 
long run it will cause nothing but trouble.

>         def __init__(self, x, y, z = 0):
>                 self.position = [x, y, z]
> 
> class Line:
>         points = ()
>         def __init__(self, p_i, p_f):
>                 self.points = (p_i, p_f)
> 
> class Model:
>         points = []
>         lines = []
> 
> 
> </code>
> 
> It would be nice if, whenever a Point object was updated, the Line objects
> which are associated with it could reflect those updates directly in
> Line.p_i and Line.p_f.
> 
> What's the Python way of achieving the same effect?

Your code already does that. Here's a minimally cleaned up version:

$ cat tmp_points.py
class Point:
    def __init__(self, x, y, z=0):
        self.position = (x, y, z)
    def __str__(self):
        return "%s" % (self.position,)

class Line:
    def __init__(self, p_i, p_f):
        self.points = (p_i, p_f)
    def __str__(self):
        return "%s-->%s" % self.points

a = Point(1, 2, 3)
b = Point(4, 5, 6)
ab = Line(a, b)
print(ab)
a.position = (10, 20, 30)
print(ab)

$ python3 tmp_points.py 
(1, 2, 3)-->(4, 5, 6)
(10, 20, 30)-->(4, 5, 6)

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Thread

Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 14:18 +0100
  Re: Newbie: question regarding references and class relationships Roy Smith <roy@panix.com> - 2013-06-10 09:35 -0400
    Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 14:57 +0100
      Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 15:00 +0100
      Re: Newbie: question regarding references and class relationships Ian Kelly <ian.g.kelly@gmail.com> - 2013-06-10 13:56 -0600
        Re: Newbie: question regarding references and class relationships Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-11 17:02 -0700
  Re: Newbie: question regarding references and class relationships Peter Otten <__peter__@web.de> - 2013-06-10 15:49 +0200
    Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 14:59 +0100
      Re: Newbie: question regarding references and class relationships Peter Otten <__peter__@web.de> - 2013-06-10 16:28 +0200
        Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 15:38 +0100
          Re: Newbie: question regarding references and class relationships Peter Otten <__peter__@web.de> - 2013-06-10 17:05 +0200
            Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 16:20 +0100
              Re: Newbie: question regarding references and class relationships Peter Otten <__peter__@web.de> - 2013-06-10 17:55 +0200
                Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 17:09 +0100
                Re: Newbie: question regarding references and class relationships Peter Otten <__peter__@web.de> - 2013-06-10 19:07 +0200
                Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 18:42 +0100
                Re: Newbie: question regarding references and class relationships Dave Angel <davea@davea.name> - 2013-06-10 15:36 -0400
                Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 21:31 +0100
                Re: Newbie: question regarding references and class relationships Terry Jan Reedy <tjreedy@udel.edu> - 2013-06-10 15:51 -0400
                Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 21:13 +0100
                Re: Newbie: question regarding references and class relationships Terry Jan Reedy <tjreedy@udel.edu> - 2013-06-10 18:07 -0400
                Re: Newbie: question regarding references and class relationships Grant Edwards <invalid@invalid.invalid> - 2013-06-10 22:39 +0000
                Re: Newbie: question regarding references and class relationships Chris Angelico <rosuav@gmail.com> - 2013-06-11 08:54 +1000
                Re: Newbie: question regarding references and class relationships Tim Chase <python.list@tim.thechases.com> - 2013-06-10 18:24 -0500
                Re: Newbie: question regarding references and class relationships Dave Angel <davea@davea.name> - 2013-06-10 19:26 -0400
                Re: Newbie: question regarding references and class relationships Jason Swails <jason.swails@gmail.com> - 2013-06-10 23:49 -0400
  Re: Newbie: question regarding references and class relationships Terry Jan Reedy <tjreedy@udel.edu> - 2013-06-10 15:54 -0400
    Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-10 21:09 +0100
  Re: Newbie: question regarding references and class relationships Rick Johnson <rantingrickjohnson@gmail.com> - 2013-06-11 16:50 -0700
    Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-13 13:06 +0100
      Re: Newbie: question regarding references and class relationships Chris Angelico <rosuav@gmail.com> - 2013-06-14 00:07 +1000
        Re: Newbie: question regarding references and class relationships Rui Maciel <rui.maciel@gmail.com> - 2013-06-13 19:23 +0100

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