Remove duplicates in list of object with Python

Question

I've got a list of objects and I've got a db table full of records. My list of objects has a title attribute and I want to remove any objects with duplicate titles from the list (leaving the original).

Then I want to check if my list of objects has any duplicates of any records in the database and if so, remove those items from list before adding them to the database.

I have seen solutions for removing duplicates from a list like this: myList = list(set(myList)), but i'm not sure how to do that with a list of objects?

I need to maintain the order of my list of objects too. I was also thinking maybe I could use difflib to check for differences in the titles.

Answer 1

The set(list_of_objects) will only remove the duplicates if you know what a duplicate is, that is, you'll need to define a uniqueness of an object.

In order to do that, you'll need to make the object hashable. You need to define both __hash__ and __eq__ method, here is how:

http://docs.python.org/glossary.html#term-hashable

Though, you'll probably only need to define __eq__ method.

EDIT: How to implement the __eq__ method:

You'll need to know, as I mentioned, the uniqueness definition of your object. Supposed we have a Book with attributes author_name and title that their combination is unique, (so, we can have many books Stephen King authored, and many books named The Shining, but only one book named The Shining by Stephen King), then the implementation is as follows:

def __eq__(self, other):     return self.author_name==other.author_name\            and self.title==other.title 

Similarly, this is how I sometimes implement the __hash__ method:

def __hash__(self):     return hash(('title', self.title,                  'author_name', self.author_name)) 

You can check that if you create a list of 2 books with same author and title, the book objects will be the same (with is operator) and equal (with == operator). Also, when set() is used, it will remove one book.

EDIT: This is one old anwser of mine, but I only now notice that it has the error which is corrected with strikethrough in the last paragraph: objects with the same hash() won't give True when compared with is. Hashability of object is used, however, if you intend to use them as elements of set, or as keys in dictionary.

Answer 2

Since they're not hashable, you can't use a set directly. The titles should be though.

Here's the first part.

seen_titles = set() new_list = [] for obj in myList:     if obj.title not in seen_titles:         new_list.append(obj)         seen_titles.add(obj.title) 

You're going to need to describe what database/ORM etc. you're using for the second part though.