如何使用父__new__中的一些魔术来创建子类实例？ [英] How to create child class instance using some magic in parent __new__?

问题描述

例如，在以下层次结构中创建自定义数字类型

For example, creating custom number types in following hierarchy

• Number
  
  
  
  • 复杂
  
  
  • Real
    
    
    
    • Int
    
    
    • Float
    
    
    • Number
      • Complex
      • Real
        • Int
        • Float

        在 __ new __ 方法中具有逻辑：

        class Number:
            def __new__(cls, value):
                if isinstance(value, complex):
                    return Complex(value)
                elif isinstance(value, (int, float)):
                    return Real(value)
                else:
                    raise TypeError('Ну ты и мудак!!!')
        
            def __init__(self, value):
                self.value = value
        
        
        class Complex(Number):
            pass
        
        
        class Real(Number):
            def __new__(cls, value):
                if isinstance(value, int):
                    return Int(value)
                elif isinstance(value, float):
                    return Float(value)
                else:
                    raise TypeError('Ты больше не можешь вести себя так!!!')
        
        
        class Int(Real):
            pass
        
        
        class Float(Real):
            pass
        

        然后，使用 Number 构造函数创建 Float 实例：

        And then, creating Float instance using Number constructor:

        number = Number(6.6)
        assert isinstance(number, Float)
        

        由于递归限制，此代码将引发 RuntimeError 。这只是期望行为的示例。但是我也尝试使用 super 来实现这一点，似乎我误解了世界。

        This code will raise RuntimeError because of recursion limit. It is just example of desired behaviour. But I also tried to implement this using super and it is seems I misunderstand the world.

        推荐答案

        您的复杂，浮动和 Int 类不是不是具有 __ new __ 方法，因此它们从 Number 继承这些方法；这就是导致您无限递归的原因。

        Your Complex, Float and Int classes do not have __new__ methods, so they inherit these from Number; this is what is causing your infinite recursion.

        您可以针对 __ class __ 闭包进行测试，以查看是否具有是否子类：

        You could test against the __class__ closure to see if you have a subclass or not:

        class Number:
            def __new__(cls, value):
                if cls is not __class__:
                    # Subclass, create an instance (invokes object.__new__)
                    return super().__new__(cls)
        
                if isinstance(value, complex):
                    return Complex(value)
                elif isinstance(value, (int, float)):
                    return Real(value)
                else:
                    raise TypeError('Ну ты и мудак!!!')
        
            def __init__(self, value):
                self.value = value
        

        并在 Real 中执行相同操作：

        class Real(Number):
            def __new__(cls, value):
                if cls is not __class__:
                    # Subclass, create an instance
                    return super().__new__(cls, value)
        
                if isinstance(value, int):
                    return Int(value)
                elif isinstance(value, float):
                    return Float(value)
                else:
                    raise TypeError('Ты больше не можешь вести себя так!!!')
        

        演示：

        >>> class Number:
        ...     def __new__(cls, value):
        ...         if cls is not __class__:
        ...             # Subclass, create an instance
        ...             return super().__new__(cls)
        ...         if isinstance(value, complex):
        ...             return Complex(value)
        ...         elif isinstance(value, (int, float)):
        ...             return Real(value)
        ...         else:
        ...             raise TypeError('Ну ты и мудак!!!')
        ...     def __init__(self, value):
        ...         self.value = value
        ... 
        >>> class Real(Number):
        ...     def __new__(cls, value):
        ...         if cls is not __class__:
        ...             # Subclass, create an instance
        ...             return super().__new__(cls, value)
        ...         if isinstance(value, int):
        ...             return Int(value)
        ...         elif isinstance(value, float):
        ...             return Float(value)
        ...         else:
        ...             raise TypeError('Ты больше не можешь вести себя так!!!')
        ... 
        >>> class Complex(Number):
        ...     pass
        ... 
        >>> class Int(Real):
        ...     pass
        ... 
        >>> class Float(Real):
        ...     pass
        ... 
        >>> number = Number(6.6)
        >>> isinstance(number, Float)
        True
        

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