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python - 自动生成重载运算符？

问题描述

考虑以下在 Python 中具有所有向量操作的向量对象的实现：

import operator

class Vector:

    def __init__(self, value):
        self._vals = value.copy()


    @classmethod
    def _op(cls, this, that, oper, rev=False):

        assert isinstance(this, cls)

        if rev:
            op = lambda a, b : oper(b, a)
        else:
            op = oper

        if isinstance(that, list):
            result = [op(x, y) for (x, y) in zip(this._vals, that)]
        elif isinstance(that, cls):
            result = [op(x, y) for (x, y) in zip(this._vals, that._vals)]
        else:
            # assume other is scalar
            result = [op(x, that) for x in this._vals]

        return cls(result)

    def __add__(self, other):
        return Vector._op(self, other, operator.add, False)


    def __radd__(self, other):
        return Vector._op(self, other, operator.add, True)

    def __sub__(self, other):
        return Vector._op(self, other, operator.sub, False)

    def __rsub__(self, other):
        return Vector._op(self, other, operator.sub, True)

    def __mul__(self, other):
        return Vector._op(self, other, operator.mul, False)

    def __rmul__(self, other):
        return Vector._op(self, other, operator.mul, True)

    def __truediv__(self, other):
        return Vector._op(self, other, operator.truediv, False)

    def __rtruediv__(self, other):
        return Vector._op(self, other, operator.truediv, True)

    def __str__(self):
        return str(self._vals)

很明显，所有重载的运算符 ( __add__, __radd__, ...) 都有完全相同的代码，除了传递给私有类方法的operand参数。 rev_op

有没有办法避免所有的复制粘贴和自动（或半自动）这些操作？

标签： pythonoperator-overloading

解决方案

第一件事：类方法也可用于实例，因此您应该将方法编写为

def __someop__(self, other):
    return self._op(self, other, ...)

或者，很容易获得一个对象的cls = type(obj)类型_op（

现在问您的问题：从技术上讲，您可以使用自定义元类或类装饰器来创建方法，即：

class VectorType(type):
    def __new__(meta, name, bases, attrs):
        todo = [
            # name, operator, reverse
            ("__add__", operator.add, False), 
            ("__sub__", operator.sub, False),
            # etc
            ]
         for name, oper, rev in todo:
             # avoids overriding already defined methods
             if name not in attrs:
                 def f(self, other):  
                     return cls._op(self, other, oper=oper, rev=rev)
                 attrs[name] = f

          return type.__new__(meta, name,   bases, attr) 

class Vector(metaclass=VectorType):
    # etc

但这确实不会改善您现有的代码，除非您有很多（不相关的）类使用此方案，即使这样，普通的 mixin 类也可以正常工作。

如果您只有这个类的问题，那么您当前的代码是清晰、简单、易读和明确的，IOW 完全是 pythonic。

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