Python Inheritance question

Question

I have a class of boxes that need to move around in a grid, but need to access that grid's coordinates in its methods. I don't want my classes to inherit the grid unless they need to. Do I need to pass the grid to the classes as a parameter, or make the box classes inherit from the grid class? Thanks.

This is the edit. Let me clarify. The classes of boxes need to know the length and width of the grid. Therefore, I need to call inside the class functions like Grid.length() and stuff. I need it though, to give me the length and width of the grid that was created.

What do you mean by "access that grid's coordinates?" Do they need to know their own coordinates within the grid? Or some other coordinates? — johnsyweb, Jun 08 '11 at 03:52

cwallenpoole · Accepted Answer · 2011-06-08T03:55:37.590

3

General rule would be parameter.

The question is is-a vs. has-a:

 Is the box a grid? 
     yes: # if so, then is it really a box???
         The box should inherit grid
     no:
         The box should get a reference to the grid 
            #(generally through a setter or constructor parameter).

Edit

An example:

class Grid:
    def __init__(self,width=1,height=1):
        this.width = width; this.height = height;

    def getDimensions(self):
        return (this.width, this.height)

class Box:
    def __init__(self, grid):
        this.__grid = grid; this.x = 0; this.y = 0            

    def verify(self):
        width, height = this.__grid.dimensions()
        if this.x < width and this.y < height:
             print( "A-OK!" ); 
        else
             print( "I am off the grid!!!" )


grid = Grid();
box = Box(grid);
box.verify();

edited Jun 08 '11 at 03:55

answered Jun 08 '11 at 03:43

cwallenpoole

79,954
26
128
166

the box is not a grid, it is a ball inside the grid. So how would I pass it as a parameter? – TheChes44 Jun 08 '11 at 03:45
Question: why are you using this? In 3.2, don't you have to use self._insertvariablehere – TheChes44 Jun 08 '11 at 03:59
By "`this`" do you mean "`self`"? – johnsyweb Jun 08 '11 at 04:47

score 0 · Answer 2 · edited May 23 '17 at 12:10

0

The box should reference to the grid. However, with inner classes, you don't need to pass the grid as a parameter, with inner classes. Inner classes is a Java feature, but you can metaprogram it to Python.

http://tomayko.com/writings/python-inner-classes

What is the purpose of python's inner classes?

edited May 23 '17 at 12:10

Community

1
1

answered Jun 08 '11 at 03:46

Ming-Tang

17,410
8
38
76

Does that speed up the process at all? Or would it be just the same as giving the grid as a parameter to one of the classes, then inheriting from that class? – TheChes44 Jun 08 '11 at 03:50
well, because I am in a time crunch, I am not worried about speed, just that it works at the moment. – TheChes44 Jun 08 '11 at 03:57

score 0 · Answer 3 · answered Jun 08 '11 at 03:47

0

Since a box is not a refinement of a type of grid you should not inherit them.

You may be interested in the Acquisition pattern and module. This allows getting attributes from the dynamic containment hierarchy, rather than static class (is-a) hierarchy.

answered Jun 08 '11 at 03:47

Keith

42,110
11
57
76

It's not clear and I haven't tried it on 3.x. Try it and see. :-) – Keith Jun 08 '11 at 03:52

score 0 · Answer 4 · edited May 23 '17 at 10:34

0

From what you have said, box classes should not inherit from the grid class as this would violate the LSP.

See: What is the Liskov Substitution Principle?

The box classes should know how to query the grid for relevant information.

edited May 23 '17 at 10:34

Community

1
1

answered Jun 08 '11 at 03:59

johnsyweb

136,902
23
188
247

Python Inheritance question

4 Answers4

Edit