Adding up instance variables of a class object

Question

I want to sum up an instance variable of a class object in a list of that class object.

Class A(object):

    def __init__(self):
        self.a = 20

B = []
for i in range(10):
    B.append(A())

# Can this be made more pythonic?
sum = 0 
for i in B:
    sum += i.a

I was thinking along the lines of using a map function or something? Don't know if that's pushing it too much though. Just curious.

sum(i.a for i in B) (See: the tutorials on list comprehensions and generator expressions.) — millimoose
– millimoose, Commented Sep 5, 2013 at 0:44
(As much as I dislike abusing the term "pythonic", I'd argue that list comprehensions are clearly more so than map() and filter().) — millimoose
– millimoose, Commented Sep 5, 2013 at 0:47
@millimoose..This is actually giving me a TypeError saying 'int' object is not callable — Nitin
– Nitin, Commented Sep 5, 2013 at 0:50
@Nitin That's probably because you still have sum = 0 somewhere in your script. This shows why it's bad to name your variables sum. — flornquake
– flornquake, Commented Sep 5, 2013 at 0:56
@Nitin The error is somewhere else than in my suggestion: ideone.com/Dx7v2I (sum() is a builtin function in it, not your variable.) — millimoose
– millimoose, Commented Sep 5, 2013 at 0:57

Bor · Accepted Answer · 2013-09-05 00:58:00Z

3

class A(object):

def __init__(self):
    self.a = 20

B = []
for i in range(10):
    B.append(A())

s = sum(i.a for i in B)

print s

works.

answered Sep 5, 2013 at 0:58

Bor

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zs2020 · Accepted Answer · 2013-09-05 00:53:31Z

2

You can use reduce

reduce(lambda acc, c: acc + c, [i.a for i in B])

or sum() with comprehension

sum([i.a for i in B])

edited Sep 5, 2013 at 0:53

answered Sep 5, 2013 at 0:45

zs2020

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