Unpack list of objects into separate attributes

Question

I have a simple Python class like this:

class P(object):
    def __init__(self, x, y):
        self.x = x
        self.y = y

I create a list of these objects e.g.:

import random

list_of_objects = [P(random.random(), random.random()) for i in range(1000)]

How do I then unpack the attributes of P into different lists?

That is, my goal is to obtain list_of_x, list_of_y, where these are defined as:

list_of_x = [p.x for p in list_of_objects]
list_of_y = [p.y for p in list_of_objects]

I was wondering if there was a cleaner way e.g. using zip/tuples, rather than the code above, which involves 2 for-loops.

Looks clean and fast to me. Likely faster than Terry's answer. — superb rain, Feb 19 '21 at 13:34

milembar · Answer 1 · 2021-02-19T13:45:40.903

0

There is always several ways to write something in a one-liner in Python, but I would not recommend it at all:

list_of_x, list_of_y = map(list, zip(*[p.__dict__.values() for p in list_of_objects]))

or more explicit:

list_of_x, list_of_y = map(list, zip(*[(p.x, p.y) for p in list_of_objects]))

Yet what you are doing is both clear and efficient, so I see no reason to write it in a one-liner. The above-mentioned implementations are not efficient since it involves additional copies to convert the list of pairs in two independent tuples, then to convert tuples in lists.

edited Feb 19 '21 at 13:45

answered Feb 19 '21 at 13:21

milembar

919
13
17

This still ends up doing 2 iterations(1 for the list comp, 1 for zip/unpack) and you end up with tuples, not lists. – Terry Spotts Feb 19 '21 at 13:45
Indeed, but the question was about using a single for-loop. Fixed tuple type issue using infamous `map`. – milembar Feb 19 '21 at 13:46
Bit shorter way to convert to lists, rather for fun: `[*list_of_x], [*list_of_y] = zip(...)`. – superb rain Feb 19 '21 at 13:55

score 0 · Answer 2 · answered Feb 19 '21 at 13:32

0

Just use 1 for loop and append the x/y to empty lists:

list_of_x = []
list_of_y = []
for p in list_of_objects:
    list_of_x.append(p.x)
    list_of_y.append(p.y)

answered Feb 19 '21 at 13:32

Terry Spotts

3,527
1
8
21

Not sure that's good advice, given how it spreads creation over multiple separate statements (so I wouldn't call it "cleaner") and appears to take about [twice as much time](https://tio.run/##fVHLbsMgELzzFXsECVmJcqkq5R9y6C2KELEh3co8hIlkvt4FY9dKmnYviJllZjT4FD@dPbz5ME06OAMRjcIIaLwLEYLySkay3qTtnCGk7eUwwIm665dqI3snkKdTGoRAi1EIOqhecxg5pIUtU8BmhCOMj1DKUCKkxyEKp0VVHTJ4PtFq2dSDMg5PAAPtAiCgLcxN0f1ut2MXQkocF/CGVvZ0CbE6lAxnn6OUt768ffK@PKynup7@WZ/tPlQI6aXXC70K/aW4lfaj00jvle1ozs1@sWljEyOkyIqtk8OSqcD6btvCrN3wGnszjDmdQUvr19Oyz8HezVWF41zu5u4D2kgjn0UbIaw0SojKV46RafoG). – superb rain Feb 19 '21 at 13:44

Unpack list of objects into separate attributes

2 Answers2