Fast removal of consecutive duplicates in a list and corresponding items from another list

Question

My question is similar to this previous SO question. I have two very large lists of data (almost 20 million data points) that contain numerous consecutive duplicates. I would like to remove the consecutive duplicate as follows:

list1 = [1,1,1,1,1,1,2,3,4,4,5,1,2]  # This is 20M long!
list2 = ...  # another list of size len(list1), also 20M long!
i = 0
while i < len(list)-1:
    if list[i] == list[i+1]:
        del list1[i]
        del list2[i]
    else:
        i = i+1

And the output should be [1, 2, 3, 4, 5, 1, 2] for the first list. Unfortunately, this is very slow since deleting an element in a list is a slow operation by itself. Is there any way I can speed up this process? Please note that, as shown in the above code snipped, I also need to keep track of the index i so that I can remove the corresponding element in list2.

Answer 1

Python has this groupby in the libraries for you:

>>> list1 = [1,1,1,1,1,1,2,3,4,4,5,1,2]
>>> from itertools import groupby
>>> [k for k,_ in groupby(list1)]
[1, 2, 3, 4, 5, 1, 2]

You can tweak it using the keyfunc argument, to also process the second list at the same time.

>>> list1 = [1,1,1,1,1,1,2,3,4,4,5,1,2]
>>> list2 = [9,9,9,8,8,8,7,7,7,6,6,6,5]
>>> from operator import itemgetter
>>> keyfunc = itemgetter(0)
>>> [next(g) for k,g in groupby(zip(list1, list2), keyfunc)]
[(1, 9), (2, 7), (3, 7), (4, 7), (5, 6), (1, 6), (2, 5)]

If you want to split those pairs back into separate sequences again:

>>> zip(*_)  # "unzip" them
[(1, 2, 3, 4, 5, 1, 2), (9, 7, 7, 7, 6, 6, 5)]