Question

Background

I have a list.
This list has many objects. Each object has an id. Now the objects are of different types.

objects = [Aobject, Bobject, Cobject]

where

>>> Aobject != Bobject
True
>>> Aobject.id ==  Bobject.id
True

Problem

I want a list of unique objects based on the object.id.

Something like this:

set(objects, key=operator.attrgetter('id'))

(This does not work. But I want something like this)

Answer 1

You can use a set:

seen = set()
uniqueidlist = []
for obj in mylist:
    if obj.id not in seen:
        seen.add(obj.id)
        uniqueidlist.append(obj)

Or equivalently:

seen = set()
uniqueidlist = [seen.add(obj.id) or obj for obj in mylist if obj.id not in seen]

This works because set.add returns None, so the expression in the list comprehension always yields obj, but only if obj.id has not already been added to seen.

The expression could only evaluate to None if obj is None; in that case, obj.id would raise an exception. In case mylist contains None values, change the test to if obj and (obj.id not in seen).

Note that for each obj.id, this will keep the first obj in the list which has that obj.id.

Update

Alternatively, you can use a dict:

seen = {}
for obj in mylist:
    if obj.id not in seen:
        seen[obj.id] = obj
uniqueidlist = list(seen.values())

Note that for each obj.id, this will keep the first obj in the list which has that obj.id. Remove the test if you want to keep the last such obj, or use @Abhijit’s answer.