How to Merge and Filter Python Lists Using itertools.chain and List Comprehensions

1. Introduction

In a Python discussion group a user asked how to process several nested lists to extract sub‑lists that contain any of the characters 'a', 'b', or 'c'. The article presents a step‑by‑step solution.

2. Original Data

The three source lists are:

l1 = [['a', 'b', 'c'], [1, 2, 3], ['c', 'c', 'c']]
l2 = [[1, 1, 1], ['a', 'a', 'a'], ['c', 0, 0, 0]]
l3 = [['c', 'c', 'c'], [5, 6, 7], [7, 3, 4], [0, 'a']]

These lists are visualised in the original article (image omitted for brevity).

3. First Implementation

The solution uses itertools.chain to iterate over all sub‑lists and a break statement to stop after the first matching element, thereby avoiding duplicate entries:

from itertools import chain
l = []
for i in chain(l1, l2, l3):
    for j in i:
        if j in ('a', 'b', 'c'):
            l.append(i)
            break
print(l)

The output shows the sub‑lists that contain at least one of the target characters.

4. List‑Comprehension Alternative

A more compact version combines the three lists, then uses a nested list comprehension to collect matching sub‑lists and a second pass to remove duplicates:

all_list = l1 + l2 + l3
l = []
res = [l.append(i) for i in all_list for j in i if j in ('a', 'b', 'c')]
final_res = []
result = [final_res.append(item) for item in l if item not in final_res]
print(final_res)

Although slightly redundant, this code produces the same correct result.

5. Conclusion

The article demonstrates two practical ways to merge and filter Python lists, helping the original asker solve the problem efficiently.