Question

我正在尝试实现contains，它可以用于我的python对象中的任何几个属性。我能够成功实施＆＃34; ==＆＃34;和大多数其他比较运算符，但＆＃34; in＆＃34;给我带来了问题：

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)
         def __contains__(self,other):
             return Comparator(self.name,other,operator.contains)
    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)

# the line below works fine!
print list(mytable.filter(Row.val3 == 7))

# this line below does not work
print list(mytable.filter("h" in Row.val4))
# TypeError: 'bool' object is not callable

# this line also does not work
print list(mytable.filter(Row.val4 in "hello world"))
# TypeError: 'in <string>' requires string as left operand, not instance

Answer 1

对于filter，你必须传递一个可调用的，而不是一个布尔值
row_obj.val4是类RowItem的一个实例，而不是__contains__类的string方法所期望的字符串

Answer 2

感谢Kevin在评论中回答这个问题。问题是in（__contains__()方法）将结果强制转换为布尔值，与其他逻辑比较运算符（__lt__()，__eq__()和其他运算符不同。）

似乎这样做的原因主要是向后兼容性。更多信息： https://mail.python.org/pipermail/python-dev/2013-July/127297.html

解决此问题的一种方法是创建一个新方法（例如，包含_）：

尝试这样的事情（这是一个不好的例子，因为包含可以在这段代码中使用：

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)
         def contains_(self,other):
             return Comparator(self.name,other,operator.contains)
    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)

而不是：

         def __contains__(self,other):
             return Comparator(self.name,other,operator.contains)

当然，在尝试执行“in”时，您需要执行以下操作：

print list(mytable.filter(Row.val4.contains_("h"))) #new way to call in (__contains__)

而不是：

print list(mytable.filter(Row.val4.__contains__("h"))) #old broken way to call in (__contains__)

为Python类实现contains （in）时出错

2 个答案:

为Python类实现__contains __（in）时出错

2 个答案:

为Python类实现contains （in）时出错