How Can I Efficiently Remove Consecutive Duplicate Elements from a Python List?

Removing Elements with Consecutive Duplicates

A common task in data processing is eliminating consecutive duplicates from a list. A straightforward Python implementation might be:

list = [1,1,1,1,1,1,2,3,4,4,5,1,2]
i = 0

while i < len(list)-1:
    if list[i] == list[i+1]:
        del list[i]
    else:
        i = i+1

This approach removes repeated elements, resulting in an output like [1, 2, 3, 4, 5, 1, 2].

Eliminating Elements Whose Values Repeat Consecutively

However, an improved goal is to remove entire elements whose values repeat consecutively, resulting in an output like [2, 3, 5, 1, 2]. The previous approach can be modified:

list = [1,1,1,1,1,1,2,3,4,4,5,1,2]
i = 0
dupe = False

while i < len(list)-1:
    if list[i] == list[i+1]:
        del list[i]
        dupe = True
    elif dupe:
        del list[i]
        dupe = False
    else:
        i += 1

While functional, this approach could be simplified.

A More Elegant Solution

Python provides more expressive tools for list manipulation. Using itertools.groupby:

L = [1,1,1,1,1,1,2,3,4,4,5,1,2]
from itertools import groupby
[key for key, _group in groupby(L)]

This yields the desired output: [1, 2, 3, 4, 5, 1, 2].

For the second part of the task:

[k for k, g in groupby(L) if len(list(g)) < 2]

This uses groupby to group consecutive duplicates and filters out groups with more than one element, effectively removing the duplicated elements.

If desired, you can avoid creating a temporary list by using a generator expression:

[k for k, g in groupby(L) if sum(1 for i in g) < 2]

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