在python中實現多個構造函數的最佳實踐

Question

我非常肯定這已經被問了很多次，但我仍然不確定如何在Python中實現多個構造函數。我知道在python中，我只能有一個不同於java或C＃或C++的構造函數。我對它仍然很陌生。長話短說，我需要實現一個線對象。該行將由函數y = ax + b表示。所以我需要在行中存儲的唯一東西是a，b和布爾值，用於垂直線（a =無窮大）的特殊情況。在這種情況下，a將存儲該行的x位置。爲了創建一條線，我有3種方法。 1是直接放入a，b和布爾值。 2是以元組的形式放2分。 3是放在一個點和一個向量。到目前爲止我的代碼：

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來做到這一點

Answer 1

UPDATE：

做多個構造的更多Python的方式是@classmethod，由Jim的建議。 Raymond Hettinger在Pycon 2013上就Python的班級開發工具包進行了演講，在那裏他使用@classmethod談論了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類似，cls參數爲@classmethod用作調用類隱式傳遞（在上面的例子中，這將是Line）。這用於使用其他構造函數來適應未來的子類;它通過硬編碼基類來代替cls，從而避免了意外繞過子類實現構造函數的潛在錯誤。

---

原貼：

如果要強制使用您的構造函數，你可以讓他們static methods，並讓他們回到你的類的實例。

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)) 

編輯：
如果要強制使用您的構造函數在使用Line.__init__，您可以使用下面的工廠隱藏線類直接實例：

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

您可以創建一個構造函數所需的所有參數的默認值無：

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

然後你會檢查哪些paameters在構造函數中傳遞並從這些參數創建行。

Answer 3

你可以使用Pyhton的枚舉來聲明初始化方法如下：

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) 

然後，創建一個如下所示的行：

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