Python 类和类型

Question

我想我误用了 subclass 的概念。我正在使用 Grids and Cells 开展一个业余爱好项目。

我所拥有的是 Cell class 及其子 class HexCell 的实现，它基本上重新定义了许多 attributes/methods所以：

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)

然后我有一个 subclass，即 HexGrid，它遵循具有新参数的类似结构。

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)

在这种情况下，方法self.link(other)是共享的，但其他属性从矩形网格变为六边形，如位置从(row, col)变为(hex_r, hex_th)，或neighbors作为 4 列表或 6 列表。因此，我希望这些属性依赖于另一个单元格类型属性并向下传输到 subclass。

Answer 1

subclassing的正确使用需要遵守以下替换原则：

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.

换句话说，您希望 subclassing 实施新行为，而不是改变现有行为。

在你的例子中，坐标系的改变本身就是行为的改变，因此HexCell不应该继承自Cell。

你可以做的是创建一个基础 class BaseCell 来封装 Cell 和 HexCell 之间的共同行为并从中继承。

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):
        ...

Answer 2

你的单元格class其实不是抽象的"Cell"，而是二维space中的方形单元格（恰好有4个邻居，有"row"和"col" 位置）。这样的单元格不能被十六进制单元格子class，因为十六进制单元格只是一种不同类型的单元格:)

如您所见，唯一常见的是 link() 方法和 links 属性。如果你坚持使用 subclassing，你可以创建如下内容：

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

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Python 类和类型