I have a list of tuples (always pairs) like this:

``````[(0, 1), (2, 3), (5, 7), (2, 1)]
``````

I’d like to find the sum of the first items in each pair, i.e.:

``````0 + 2 + 5 + 2
``````

How can I do this in Python? At the moment I’m iterating through the list:

``````sum = 0
for pair in list_of_pairs:
sum += pair
``````

I have a feeling there must be a more Pythonic way.

In modern versions of Python I’d suggest what SilentGhost posted (repeating here for clarity):

``````sum(i for i, j in list_of_pairs)
``````

In an earlier version of this answer I had suggested this, which was necessary because SilentGhost’s version didn’t work in the version of Python (2.3) that was current at the time:

``````sum([pair for pair in list_of_pairs])
``````

Now that version of Python is beyond obsolete, and SilentGhost’s code works in all currently-maintained versions of Python, so there’s no longer any reason to recommend the version I had originally posted.

``````sum(i for i, j in list_of_pairs)
``````

will do too.

I recommend:

``````sum(i for i, _ in list_of_pairs)
``````

Note:

Using the variable `_`(or `__` to avoid confliction with the alias of `gettext`) instead of `j` has at least two benefits:

1. `_`(which stands for placeholder) has better readability
2. `pylint` won’t complain: “Unused variable ‘j'”

If you have a very large list or a generator that produces a large number of pairs you might want to use a generator based approach. For fun I use `itemgetter()` and `imap()`, too. A simple generator based approach might be enough, though.

``````import operator
import itertools

idx0 = operator.itemgetter(0)
list_of_pairs = [(0, 1), (2, 3), (5, 7), (2, 1)]
sum(itertools.imap(idx0, list_of_pairs))
``````

Note that `itertools.imap()` is available in Python >= 2.3. So you can use a generator based approach there, too.

Obscure (but fun) answer:

``````>>> sum(zip(*list_of_pairs))
9
``````

Or when zip’s are iterables only this should work:

``````>>> sum(zip(*list_of_pairs).__next__())
9
``````

Below is sample code, you can also specify the list range.

``````def test_lst_sum():
lst = [1, 3, 5]
print sum(lst)  # 9
print sum(lst[1:])  # 8

print sum(lst[5:])  # 0  out of range so return 0
print sum(lst[5:-1])  # 0

print sum(lst[1: -1])  # 3

lst_tp = [('33', 1), ('88', 2), ('22', 3), ('44', 4)]
print sum(x for x in lst_tp[1:])  # 9
``````

If you don’t mind converting it to a numpy array, you can use `np.sum` over `axis=0` as given here

``````                s,p=0,0
for i in l:
s=s+i
p=p+i
print(tuple(s,p))
``````