Sum / Average an attribute of a list of objects

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])

Use a generator expression:

sum(c.a for c in c_list)

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.