Python merge lists by common element

Question

Im trying to merge two lists that have a common thing between them (in that case is a the id parameter). I have something like this:

list1=[(id1,host1

Answer 1

>>> from collections import defaultdict
>>> list1 = [('id1','host1'),('id2','host2'),('id1','host5'),('id3','host4'),('id4','host6'),('id5','host8')]
>>> list2 = [('id1','IP1'),('id2','IP2'),('id3','IP3'),('id4','IP4'),('id5','IP5')]
>>> d1 = defaultdict(list)
>>> for k,v in list1:
...     d1[k].append(v)
... 

You can print the items like this

>>> for k, s in list2:
...     print s, d1[k]
... 
IP1 ['host1', 'host5']
IP2 ['host2']
IP3 ['host4']
IP4 ['host6']
IP5 ['host8']

You can use a list comprehension to put the results into a list

>>> res = [(s, d1[k]) for k, s in list2]
>>> res
[('IP1', ['host1', 'host5']), ('IP2', ['host2']), ('IP3', ['host4']), ('IP4', ['host6']), ('IP5', ['host8'])]