Class instance has no attribute 'function_name' for passing object outside class- python

Question

my class structure is something like this.

class A():
    def __init__(self):
        self.matched_condition_set = set()

    def add_to_matched_condition(self,condition):
        self.matched_condition_set.add(condition)

class B():
    def __init__(self, list_of_A):
        self.list_of_a = list_of_A

    def do_stuff(self):
        for a in self.list_of_a:
            if helper.check_for_a(a):
                print(a.matched_condition_set)

in a file called helper.py i have the following function.

def check_for_a(a):
    print(type(a))
    a.add_to_matched_condition("hello")
    return True

Now if i call class B object, I get that:

A instance has no attribute 'add_to_matched_condition'

. also when I try to get the type of a >> print(type(a)) inside the helper method. i get type<'instance'>. where is the object getting lost ?

Works for me - a = A(); b = B([a]); b.do_stuff() results in <type 'instance'> and set(['hello']). — wwii
– wwii, Commented Dec 8, 2014 at 4:59

PM 2Ring · Accepted Answer · 2014-12-08 04:59:37Z

I don't know why you're getting A instance has no attribute 'add_to_matched_condition'; your code works ok for me on Python 2.6.4

To get a more useful type signature (plus other goodies) on your classes you need to make them inherit from object.

Here's my slightly modified version of your code that illustrates that; it also shows how I tested your classes.

#!/usr/bin/env python

import helper

class A(object):
    def __init__(self):
        self.matched_condition_set = set()

    def add_to_matched_condition(self,condition):
        self.matched_condition_set.add(condition)

class B(object):
    def __init__(self, list_of_A):
        self.list_of_a = list_of_A

    def do_stuff(self):
        for a in self.list_of_a:
            if helper.check_for_a(a):
                print(a.matched_condition_set)


a1 = A(); a2 = A()
b = B([a1, a2])

print a1, a2
print b, b.list_of_a

b.do_stuff()

print a1.matched_condition_set 
print a2.matched_condition_set

output

<__main__.A object at 0xb758f80c> <__main__.A object at 0xb758f84c>
<__main__.B object at 0xb758f86c> [<__main__.A object at 0xb758f80c>, <__main__.A object at 0xb758f84c>]
<class '__main__.A'>
set(['hello'])
<class '__main__.A'>
set(['hello'])
set(['hello'])
set(['hello'])

Larry Lustig · Accepted Answer · 2014-12-08 04:58:51Z

1

The reason you are seeing type<instance> is most likely that you are using Python 2.x. Because you are defining your classes as

class A():

instead of

class A(object):

you are creating "old style" classes.

As to your other problem, we'd need to see the code in which you create a list of A()s and pass that list into the constructor for B(). Please add that code if you want an answer to that part of your question.

Also, it's unclear to me why check_for_a is not a method of A(), and I'm not sure I would name a function check_for_a if it actually has the side-effect of altering the object it's checking.

answered Dec 8, 2014 at 4:58

Larry Lustig

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2 Comments

shubham Over a year ago

declaring a class to class A(object): solved it. but what is the difference between the two types ??

PM 2Ring Over a year ago

@shubham: See the docs NewClassVsClassicClass and python.org/doc/newstyle

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