CODEWITHSUNDEEP
pythonclasssubclass
Diego-MX
Diego-MX

Reputation: 2349

Python classes and types

I think I'm misusing the concept of subclass. I'm working on a hobby project with Grids and Cells.

What I have, is the implementation of a Cell class, and its subclass HexCell which basically redefines many of the attributes/methods like so: 

class Cell: 

  def __init__(self, row_loc, col_loc):
    self.row = row_loc
    self.col = col_loc
    self.links = set()
    self.neighbors = 4*[None]

  def __repr__(self):
    return f'Cell @({self.row},{self.col})'

  def link(self, other, bidir = True):
    self.links.add(other)
    if bidir: other.links.add(self)

Then I have a subclass that is the HexGrid which follows a similar structure with new parameters. 

class HexCell(Cell):
  def __init__(self, r_out, th_around):
  # I'm indexing Hex cells around a center cell 
  # instead of by rows and columns; Prefixed hex
  # as they follow the hexagon, and not regular polar coordinates. 
    self.hex_r     = r_out
    self.hex_th    = th_around
    self.neighbors = 6*[None]
    self.links     = set()

  def __repr__(self):
    return f"HexCell @[{self.hex_r}, {self.hex_th}]"

  def bind(self, other, to_dir):
    to_dir = to_dir % 6
    if (self.neighbors[to_dir] is None):
      self.neighbors[to_dir] = other
      other.neighbors[to_dir - 3] = self

    # Hexagonal grids share neighbors. 
    other_1 = other.neighbors[to_dir - 2]
    if (self.neighbors[to_dir - 1] is None) & (other_1 is not None):
      self.bind(other_1, to_dir - 1)
    other_5 = other.neighbors[to_dir - 4]
    if (self.neighbors[to_dir - 5] is None) & (other_5 is not None):
      self.bind(other_5, to_dir - 5)

In this case, the method self.link(other) is shared, but other attributes change from rectangular grid to hexagonal like the locaion from (row, col) to (hex_r, hex_th), or neighbors as a 4-list or 6-list. Thus I'd like these attributes to be dependent on a another cell-type attribute and transferred down to the subclass.

Upvotes: 0

Views: 72

Answers (2)

Olivier Melançon
Olivier Melançon

Reputation: 22324

Correct use of subclassing needs to obey the following substitution principle:

If there are some objects x_1 of type T_1 and x_2 of type T_2 such that issubclass(T_2, T_1) == True, then any property that applies to x_1 must also apply for x_2.

In other words, you expect subclassing to implement new behaviours, not to change existing behaviours.

In you example, the change of coordinate system itself is a change of behaviour and thus HexCell should not inherit from Cell.

What you can do is create a base class BaseCell that encapsulates the common behaviour between Cell and HexCell and inherit from it.

class BaseCell:
    def __init__(self):
        self.links = set()
        self.neighbors = []

    def add_neighbor(self, other):
        self.neighbors.append(other)

    def link(self, other, bidirectional=True):
        self.links.add(other)
        if bidirectional:
            other.link(self, bidirectional=False)


class Cell(BaseCell):
    def __init__(self, row_loc, col_loc):
        self.row = row_loc
        self.col = col_loc
        super().__init__()

    def __repr__(self):
        return f'Cell @({self.row},{self.col})'


class HexCell(Cell):
    def __init__(self, r_out, th_around):
        self.hex_r = r_out
        self.hex_th = th_around
        super().__init__()

    def __repr__(self):
        return f"HexCell @[{self.hex_r}, {self.hex_th}]"

    def bind(self, other, to_dir):
        ...

Upvotes: 3

jbet
jbet

Reputation: 492

Your Cell class is in fact not an abstract "Cell", but a square cell in two-dimensional space (has exactly 4 neighbours, has "row" and "col" position). Such cell may not be subclassed by a hex cell, because hex cell is just a different type of cell : )

As you noticed, the only common things are link() method and links attribute. If you insist on subclassing, you could create something like:

class LinkedObject():
    def __init__(self):
        self.links = set()

    def link(self, other, bidir = True):
        self.links.add(other)
        if bidir: other.links.add(self)

class SquareCell(LinkedObject):
    # "Cell" class here

class HexCell(LinkedObject):
    # HexCell here

Upvotes: 1

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