CODEWITHSUNDEEP
pythonclasscontains
mgoldwasser
mgoldwasser

Reputation: 15434

Error implementing __contains__ (in) for a Python class

I am trying to implement contains such that it can be used for any of several attributes in my python object. I was able to successfully implement "==" and most of the other comparison operators, but "in" is giving me problems:

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

Upvotes: 0

Views: 563

Answers (2)

mgoldwasser
mgoldwasser

Reputation: 15434

Thanks to Kevin for answering this in the comments. The issue was that in (the __contains__() method) coerces the result to a boolean unlike the other logical comparison operators (__lt__(), __eq__(), and others).

It seems like the reason for this is mostly backwards compatibility. More information here: https://mail.python.org/pipermail/python-dev/2013-July/127297.html

One way around this is to create a new method (for example, contains_):

try something like this (this is a bad example, because contains would work in this code :

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)

instead of:

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

Of course, when trying to perform "in", you would need to do something like this:

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

instead of:

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

Upvotes: 1

mutantacule
mutantacule

Reputation: 7063

  1. For filter you have to pass a callable, not a boolean
  2. row_obj.val4 is an instance of the class RowItem, and not a string as expected by the __contains__ method of the string class

Upvotes: 1

Related Questions