Filter out nested lists by the first element in them

Question

I want to filter nested lists based on the first element in each list. I have the following nested lists:

# 'nom' is duplicated
input1 = [
    ['nom', 'N', 'eye'],
    ['nom', 'N', 'face'],
    ['acc', 'E', 'computer'],
    ['dat', 'C', 'screen']
    ]

# 'acc' is duplicated
input2 = [
    ['nom', 'O', 'heart'],
    ['acc', 'O', 'root'],
    ['acc', 'C', 'life'],
    ['dat', 'E', 'evidence']
    ]

# No duplicates
input3 = [
    ['nom', 'O', 'author'],
    ['acc', 'O', 'tear'],
    ]

For each parent list, I would like to find the child lists whose first element is the same, and keep the first child list in the output. So I want to have the following output:

# From input1
# keep ['nom', 'N', 'eye']
# delete ['nom', 'N', 'face']
output1 = [
    ['nom', 'N', 'eye'],
    ['acc', 'E', 'computer'],
    ['dat', 'C', 'screen']
    ]

# From input2
# keep ['acc', 'O', 'root']
# delete ['acc', 'C', 'life']
output2 = [
    ['nom', 'O', 'heart'],
    ['acc', 'O', 'root'],
    ['dat', 'E', 'evidence']
    ]

# From input3
# keep every child list since there is no duplicate
output3 = [
    ['nom', 'O', 'author'],
    ['acc', 'O', 'tear'],
    ]

How should I achieve this with python3?

Answer 1

If you are using Python 3.6 or newer you can use the fact that dict retain the insertion order. Iterate over the list in reverse and insert it to a dict , and than reverse the values

def filter_list(lst):
    d = {lst[i][0]: lst[i] for i in range(len(lst) - 1, -1, -1)}
    return list(reversed(d.values()))

input1 = filter_list(input1)
print(input1) # [['nom', 'N', 'face'], ['acc', 'E', 'computer'], ['dat', 'C', 'screen']]