How to get unique values from a python list of objects

Question

I have a class:

class Car:
    make
    model
    year

I have a list of Cars and want to get a list of unique models among my Cars.

The list is potentially tens of thousands of items. What's the best way to do this?

Thanks.

do you mean models that are only in the list once or to shorten the list to only have one of each? — Padraic Cunningham, Sep 25 '14 at 16:24
Also: http://stackoverflow.com/questions/6926928/get-unique-items-from-list-of-lists http://stackoverflow.com/questions/479897/how-do-you-remove-duplicates-from-a-list-in-python-if-the-item-order-is-not-impo — tom10, Sep 25 '14 at 16:28
I am not sure this is a duplicate, the OP may want to find models that only appear once which the dup will not solve — Padraic Cunningham, Sep 25 '14 at 16:30

score 50 · Accepted Answer · answered Sep 25 '14 at 16:25

50

Use a set comprehension. Sets are unordered collections of unique elements, meaning that any duplicates will be removed.

cars = [...] # A list of Car objects.

models = {car.model for car in cars}

This will iterate over your list cars and add the each car.model value at most once, meaning it will be a unique collection.

answered Sep 25 '14 at 16:25

Ffisegydd

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Many people find the curlies to read - they look like dictionaries. In that context, so as an alternative `models = list(set([car.model for car in cars]))` – theodox Sep 25 '14 at 16:40
8

you pay quite some price for getting rid of the curlies, don't you? – flaschbier Sep 25 '14 at 18:55
1

Yeah I think it's because first you create the full list (with duplicates) then you add it to a set (meaning you remove the duplicates) then you put it back in a list. – Ffisegydd Sep 25 '14 at 19:02
3

Feed your clock cycles to the god of the anti-curlie... – Tatarize Mar 01 '21 at 08:45

score 2 · Answer 2 · answered Feb 19 '21 at 18:46

Add a method to the class that returns a unique list

def unique(list1):
    # intilize a null list
    unique_list = []

    # traverse for all elements
    for x in list1:
        # check if exists in unique_list or not
        if x not in unique_list:
            unique_list.append(x)

    return unique_list

Otherwise,

If you're looking to pass a dictionary of make, model year then you can use pandas dataframe in order to get the unique values.

score 1 · Answer 3 · answered Sep 25 '14 at 16:29

1

If you want to find cars that only appear once:

from collections import Counter
car_list = ["ford","toyota","toyota","honda"]
c = Counter(car_list)
cars = [model for model in c if c[model] == 1 ]
print cars
['honda', 'ford']

answered Sep 25 '14 at 16:29

Padraic Cunningham

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Yeah I read it differently (as the OP wanting the unique set of models that he had). Hopefully they'll reply and clear it up though :) – Ffisegydd Sep 25 '14 at 16:32
+1: either this is the answer the OP is looking for, or this is just a duplicate of 50 other questions. – tom10 Sep 25 '14 at 16:45

score -4 · Answer 4 · answered Aug 09 '18 at 11:07

-4

Now it depends on what you mean by unique. If you mean Exactly the same object then this should work:

def check_all_objects_unique(objects: List[Any]) -> bool:
unique_objects = set(objects)
if len(unique_objects) == len(objects):
    return True
elif len(unique_objects) < len(objects):
    return False

If by unique you mean objects with the same attribute values then other answers on here will help you.

answered Aug 09 '18 at 11:07

henryJack

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You don't answer OPs question. This just checks if all elements are unique and in a very bad way at that. simply `return len(unique_objects) == len(objects)` – R Tsch Jun 25 '19 at 16:10

How to get unique values from a python list of objects

4 Answers4