Sum / Average an attribute of a list of objects

Question

Lets say I have class C which has attribute a.

What is the best way to get the sum of a from a list of C in Python?

I've tried the following code, but I know that's not the right way to do it:

for c in c_list:
    total += c.a

score 248 · Accepted Answer · edited May 20 '19 at 14:17

248

Use a generator expression:

sum(c.a for c in c_list)

edited May 20 '19 at 14:17

Georgy

12,464
7
65
73

answered Jun 04 '12 at 10:33

phihag

278,196
72
453
469

I like the simplicity, but it seems to take half again as long as (c1.A + c2.A + c3.A) for about 10 elements where c.A is a float. – jcomeau_ictx Jan 06 '16 at 19:52

score 9 · Answer 2 · edited May 20 '19 at 14:18

9

If you are looking for other measures than sum, e.g. mean/standard deviation, you can use NumPy and do:

mean = np.mean([c.a for c in c_list])
sd = np.std([c.a for c in c_list])

edited May 20 '19 at 14:18

Georgy

12,464
7
65
73

answered Mar 13 '19 at 08:43

Heribert

613
1
9
27

score 6 · Answer 3 · edited May 20 '19 at 14:18

6

I had a similar task, but mine involved summing a time duration as your attribute c.a. Combining this with another question asked here, I came up with

sum((c.a for c in cList), timedelta())

Because, as mentioned in the link, sum needs a starting value.

edited May 20 '19 at 14:18

Georgy

12,464
7
65
73

answered Jun 24 '15 at 15:46

Bill Kidd

1,130
11
13

score 2 · Answer 4 · answered Jan 24 '20 at 19:27

2

Use built-in statistics module:

import statistics

statistics.mean((o.val for o in my_objs))

answered Jan 24 '20 at 19:27

Shital Shah

63,284
17
238
185

Sum / Average an attribute of a list of objects

4 Answers4

Linked