为 Python class 实施 "and"？

Question

我有类似的代码：

import operator
class Comparator:
     def __init__(self,fieldName,compareToValue,my_operator):
         self.op = my_operator
         self.field = fieldName
         self.comparedTo = compareToValue
     def __call__(self,row):
         my_row_val = getattr(row,self.field)
         return self.op(my_row_val,self.comparedTo)


class Row:
    class RowItem:
         def __init__(self,name):
              self.name = name
         def __eq__(self,other):
             return Comparator(self.name,other,operator.eq)
    val1 = RowItem("val1")
    val2 = RowItem("val2")
    val3 = RowItem("val3")
    val4 = RowItem("val4")
    def __init__(self, val1, val2, val3, val4):
        self.val1 = val1
        self.val2 = val2
        self.val3 = val3
        self.val4 = val4
    def __str__(self):
        return str([self.val1,self.val2,self.val3,self.val4])
    def __repr__(self):
        return str(self)


class MyTable:
    def __init__(self,rows):
        self.rows = rows
    def filter(self,condition):
        for row in self.rows:
            if condition(row):
               yield row

rows = [Row(1,2,3,"hello"),Row(1,2,7,"cat"),Row(1,2,3,"hi"),Row(7,7,7,"foo")]
mytable = MyTable(rows)

我可以成功运行过滤测试，例如：

print list(mytable.filter(Row.val3 == 7))
# prints [[1, 2, 7, 'cat'], [7, 7, 7, 'foo']]
print list(mytable.filter(Row.val2 == 2))
# prints [[1, 2, 3, 'hello'], [1, 2, 7, 'cat'], [1, 2, 3, 'hi']]

但是当我尝试使用 and 时，它无法正常工作：

print list(mytable.filter((Row.val3 == 7) and (Row.val2 == 2)))
# this only evaluates the second condition, instead of both conditions, printing:
# [[1, 2, 3, 'hello'], [1, 2, 7, 'cat'], [1, 2, 3, 'hi']]

如何让和正常工作？

Answer 1

您不能挂接到 and 和 or 逻辑运算符，因为它们 short-circuit；左边的表达式首先被求值，如果该表达式的结果决定了结果，则右边的表达式永远不会被求值。操作 returns 计算的最后一个表达式的值。

在你的例子中，(Row.val3 == 7) and (Row.val2 == 2) 表达式首先计算 (Row.val3 == 7)，并且 returns 一个没有任何特定钩子的实例，否则就是 considered a true value，因此返回右侧表达式的结果。

您可以使用& and | (bitwise AND and OR) operators here, these delegate to the object.__and__ and object.__or__钩子。这就是像 SQLAlchemy 这样的 ORM 库所做的。

对应的operator函数为operator.and_ and operator.or_.

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