在python中实现多个构造函数的最佳实践

Question

我非常肯定这已经被问了很多次，但我仍然不确定如何在Python中实现多个构造函数。我知道在python中，我只能有一个不同于java或C＃或C++的构造函数。我对它仍然很陌生。长话短说，我需要实现一个线对象。该行将由函数y = ax + b表示。所以我需要在行中存储的唯一东西是a，b和布尔值，用于垂直线（a =无穷大）的特殊情况。在这种情况下，a将存储该行的x位置。为了创建一条线，我有3种方法。 1是直接放入a，b和布尔值。 2是以元组的形式放2分。 3是放在一个点和一个向量。到目前为止我的代码：

class line: 
    def __init__(self, a, b, noSlope): 
     self.a = a 
     self.b = b 
     self.noSlope = noSlope 

    def lineFromPoints(point1, point2): 
     deltaX = point2[0] - point1[0] 
     deltaY = point2[1] - point1[1] 
     if deltaX == 0: 
      return line(point1[0], 0, True) 
     else: 
      a = deltaY/deltaX 
      b = point1[1] - a * point1[0] 
      return line(a, b, False) 

    def lineFromVector(vector, point): 
     if vector[0] == 0: 
      return line(point1[0], 0, True) 
     else: 
      a = vector[1]/vector[0] 
      b = point1[1] - a * point1[0] 
      return line(a, b, False) 

不知道是否有更好的方法来做到这一点

Answer 1

UPDATE：

做多个构造的更多Python的方式是@classmethod，由Jim的建议。 Raymond Hettinger在Pycon 2013上就Python的班级开发工具包进行了演讲，在那里他使用@classmethod谈论了multiple constructors。

class Line: 
    def __init__(self, a, b, noSlope): 
     self.a = a 
     self.b = b 
     self.noSlope = noSlope 

    @classmethod 
    def fromPoints(cls, point1, point2): 
     deltaX = point2[0] - point1[0] 
     deltaY = point2[1] - point1[1] 
     if deltaX == 0: 
      return cls(point1[0], 0, True) 
     else: 
      a = deltaY/deltaX 
      b = point1[1] - a * point1[0] 
      return cls(a, b, False) 

    @classmethod 
    def fromVector(cls, vector, point): 
     if vector[0] == 0: 
      return cls(point1[0], 0, True) 
     else: 
      a = vector[1]/vector[0] 
      b = point1[1] - a * point1[0] 
      return cls(a, b, False) 


line = Line.fromPoints((0,0), (1,1)) 

到self类似，cls参数为@classmethod用作调用类隐式传递（在上面的例子中，这将是Line）。这用于使用其他构造函数来适应未来的子类;它通过硬编码基类来代替cls，从而避免了意外绕过子类实现构造函数的潜在错误。

---

原贴：

如果要强制使用您的构造函数，你可以让他们static methods，并让他们回到你的类的实例。

class line: 
    def __init__(self, a, b, noSlope): 
     self.a = a 
     self.b = b 
     self.noSlope = noSlope 

    @staticmethod 
    def lineFromPoints(point1, point2): 
     deltaX = point2[0] - point1[0] 
     deltaY = point2[1] - point1[1] 
     if deltaX == 0: 
      return line(point1[0], 0, True) 
     else: 
      a = deltaY/deltaX 
      b = point1[1] - a * point1[0] 
      return line(a, b, False) 

    @staticmethod 
    def lineFromVector(vector, point): 
     if vector[0] == 0: 
      return line(point1[0], 0, True) 
     else: 
      a = vector[1]/vector[0] 
      b = point1[1] - a * point1[0] 
      return line(a, b, False) 

# Create instance of class 
myLine = line.lineFromPoints((0,0), (1,1)) 

编辑：
如果要强制使用您的构造函数在使用Line.__init__，您可以使用下面的工厂隐藏线类直接实例：

class LineFactory: 
    class Line: 
     def __init__(self, a, b, noSlope): 
      self.a = a 
      self.b = b 
      self.noSlope = noSlope 

    @staticmethod 
    def fromPoints(point1, point2): 
     deltaX = point2[0] - point1[0] 
     deltaY = point2[1] - point1[1] 
     if deltaX == 0: 
      return LineFactory.Line(point1[0], 0, True) 
     else: 
      a = deltaY/deltaX 
      b = point1[1] - a * point1[0] 
      return LineFactory.Line(a, b, False) 

    @staticmethod 
    def fromVector(vector, point): 
     if vector[0] == 0: 
      return LineFactory.Line(point1[0], 0, True) 
     else: 
      a = vector[1]/vector[0] 
      b = point1[1] - a * point1[0] 
      return LineFactory.Line(a, b, False) 

# Create line  
line = LineFactory.fromPoints((0,0), (1,1)) 

Answer 2

您可以创建一个构造函数所需的所有参数的默认值无：

class line: 
    def __init__(self, a = None, b = None, noSlope = None, point1 = None, point2 = None, vector = None, point = None): 
     pass 

然后你会检查哪些paameters在构造函数中传递并从这些参数创建行。

Answer 3

你可以使用Pyhton的枚举来声明初始化方法如下：

from enum import Enum 

class Line: 

    class InitTypes(Enum): 
     coefs, point_vector, point_point, vertical = range(4) 


    def __init__(self, a, b, method): 
     self.noSlope = False 
     if method == Line.InitTypes.coefs: 
      self.a = a 
      self.b = b 
     elif method == Line.InitTypes.point_vector: 
      # Do math 
      pass 
     elif method == Line.InitTypes.point_point: 
      # Do math 
      pass 
     elif method == Line.InitTypes.vertical: 
      self.noSlope = True 
      self.a = a 
      self.b = b 
     else: 
      assert(False) 

然后，创建一个如下所示的行：

x = Line((0,1), (3,4), Line.InitTypes.point_point)