python get second value of a tuple in a list

Question

i have following list: parent_child_list with id-tuples:

[(960, 965), (960, 988), (359, 364), (359, 365), 
(361, 366), (361, 367), (361, 368), (361, 369), 
(360, 370), (360, 371), (360, 372), (360, 373), (361, 374)]

Example: i would like to print those values which are combined with the id 960. Those would be: 965, 988

I tried to convert the list into a dict:

rs = dict(parent_child_list)

Because now i could simply say:

print rs[960]

But unfortunately i forgot that dict cannot have double values so instead of getting 965, 988 as the answer i receive only 965.

Is there any easy option to keep the double values?

Many thanks

score 5 · Accepted Answer · answered Nov 17 '15 at 14:03

You can use defaultdict to create dictionary with list as its value type, then append values.

from collections import defaultdict
l = [(960, 965), (960, 988), (359, 364), (359, 365), (361, 366), (361, 367), (361, 368), (361, 369), (360, 370), (360, 371), (360, 372), (360, 373), (361, 374)]

d = defaultdict(list)

for key, value in l:
    d[key].append(value)

score 1 · Answer 2 · answered Nov 17 '15 at 13:58

1

You can use a list comprehension to build a list, using if to filter out matching ids:

>>> parent_child_list = [(960, 965), (960, 988), (359, 364), (359, 365)]
>>> [child for parent, child in parent_child_list if parent == 960]
[965, 988]

answered Nov 17 '15 at 13:58

Peter Wood

23,859
5
60
99

score 0 · Answer 3 · answered Nov 17 '15 at 13:58

You can always iterate:

parent_child_list = [(960, 965), (960, 988), (359, 364), (359, 365),
(361, 366), (361, 367), (361, 368), (361, 369),
(360, 370), (360, 371), (360, 372), (360, 373), (361, 374)]

for key, val in parent_child_list:
    if key == 960:
        print str(val)

score 0 · Answer 4 · answered Nov 17 '15 at 14:00

0

The list comprehension

[y for (x, y) in parent_child_list if x == 960]

will give you a list of y values for tuples whose x values equal 960.

answered Nov 17 '15 at 14:00

jgloves

719
4
14

score 0 · Answer 5 · answered Nov 17 '15 at 14:02

You have been given the way to extract individual using a list comprehension or loop but you can construct the dictionary you desire for all values:

>>> d = {}
>>> for parent, child in parent_child_list:
...     d.setdefault(parent, []).append(child)
>>> d[960]
[965, 988]

Alternative to using a raw python dict, you could use a collections.defaultdict(list) and just directly append, e.g. d[parent].append(child)

python get second value of a tuple in a list

5 Answers5