Python: Add/update instance attribute of custom object in a set of objects

Question

I'd like to add an attribute to an instance which is in a set. The set of objects of the class Problem is identified by attribue uid (that's why they are hashed and compared):

class Problem:
    allowed_keys = [
        'flag1',
        'a_list_of_objects'
    ]

    def __init__(self, uid, name, **kwargs):
        self.uid = uid
        self.name = name
        self.__dict__.update((k, v) for k, v in kwargs.iteritems() if k in self.allowed_keys)

    def __eq__(self, other):
        return isinstance(other, self.__class__) and getattr(other, 'uid', None) == self.uid

    def __hash__(self):
        return hash(self.uid)

    def __ne__(self, other):
         return not self.__eq__(other)

    def __repr__(self):
         return json.dumps(self.__dict__, sort_keys=True)

All instances of this problem are added to a set.

problem_set = set()

problem1 = Problem(uid="abc123", name="name1", flag1=True)
problem_set.add(problem1)

problem2 = Problem(uid="xyz789", name="name2", flag2=False)
problem_set.add(problem2)

Now if I want to update the object problem1 with another attribute (it will never exist before), it just won't add the a_list_of_objects to the existing problem1

my_list = [{"a": "avalue", "b": "bvalue"}]
problem3 = Problem(uid="abc123", name="name1", a_list_of_objects=my_list

print list(problem_set)
# same list as before

What do I need to do in order to achieve this? Using @property and create getters and setters for each attribute of class Problem?

Moses Koledoye · Accepted Answer · 2016-10-27 10:18:13Z

2

You are not modifying the existing problem1 instance. You are creating an entirely different instance problem3 albeit equal to problem1 going by your implementation of __eq__. Instances satisifying equality are not merged or synced in the way I think you expect. Instances of your class with the same uid and thus hash value can exist freely outside a set as independent objects. Trying to add one of both to a set will drop one, not sync them.

To modify problem1 you should instead do:

problem1.a_list_of_objects = my_list

Or you could assign to a copy of my_list to avoid sharing the same list among instances:

import copy

problem1.a_list_of_objects = copy.deepcopy(my_list)

New attributes can be bound to an existing instance of your class.

edited Oct 27, 2016 at 10:18

answered Oct 27, 2016 at 10:04

Moses Koledoye

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3 Comments

dh762 Over a year ago

How do I properly merge them then (based on the uid)? I need to merge multiple attributes to the instance if it's already there, or create a new one if not. The list should be shared among instances.

Moses Koledoye Over a year ago

You need to iterate through the set and check if an instance with that uid already exists. If yes, update. If no, create a new one. You can also use in to test if an equal instance already exists, but that won't give you a reference to the instance. A set comprehension can be used with the in check - if the check passes, and a simple set.add if it fails.

dh762 Over a year ago

Thanks. Your input made me rewrite the code so that the Problem instance creation happens as a last step when all values are decided. Much simpler.

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