[Tutor] Iterating through two lists at the same time with manipulation..

[Iyer]

I have 2 lists:

List 1 has lists in it, such as

list1 = [[1,'A'],[2,'B'],[3,'C'],[4,'D']]

There is another list2 such as

list2 = [[1,'AA'],[3,'CC'], [4,'DD']]

For eg,

I wish to iterate over both the lists and produce the output 

a = [[1,'A'],[1,'AA']]
b = [[2,'B']]
c = [[3,'C'],[3,'CC']]
d = [[4,'D'],[4,'DD']]

Or [a,b,c,d] where a,b,c,d are defined above

What would be the best and quickest way to carry this out ? This list comprehension doesn't seem to get the desired result:

[[x,y] for x in list1 for y in list2 if x[0]==y[0]]

the sub-list [2,'B'] in list1 is left out. i.e, the output is 
[[[1, 'A'], [1, 'AA']], [[3, 'C'], [3, 'CC']], [[4, 'D'], [4, 'DD']]]


       
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