CODEWITHSUNDEEP
python
Melsauce
Melsauce

Reputation: 2655

How to flatten one layer of list of list of lists in python?

I have a structure of the form:

lst= [[['A'], ['B'], ['C']], [['A'], ['B'], ['D'], ['E']], [['B'], ['C']]]

I want to collapse the innermost layers of the lists such that I get the following structure:

lst= [['A', 'B', 'C'], ['A', 'B', 'D', 'E'], ['B', 'C']]

I have tried to flatten the list by doing:

[item for sublst in lst for item in sublst]

But this collapses all layers instead to a one dimensional list.

What is the most pythonic way to address this?

Upvotes: 1

Views: 954

Answers (2)

Austin
Austin

Reputation: 26039

You need to iterate over sublists and index them to get desired output (but this assumes your inner list holds only one element):

lst= [[['A'], ['B'], ['C']], [['A'], ['B'], ['D'], ['E']], [['B'], ['C']]]

result = [[y[0] for y in x] for x in lst]
# [['A', 'B', 'C'], ['A', 'B', 'D', 'E'], ['B', 'C']]

Optionally, you can use chain.from_iterable from itertools (Now this handles any number of elements in inner lists):

from itertools import chain

lst= [[['A'], ['B'], ['C']], [['A'], ['B'], ['D'], ['E']], [['B'], ['C']]]

result = [list(chain.from_iterable(x)) for x in lst]

Upvotes: 1

Martijn Pieters
Martijn Pieters

Reputation: 1122292

You need to flatten the elements of lst, not lst itself.

So use the double loop list comprehension inside another list comprehension, where the outer comprehension iterates over your top-level list:

[[item for inner in sublst for item in inner] for sublst in lst]

Since your inner-most lists each only contain a single element, you could also use indexing to get those values:

[[inner[0] for inner in sublst] for sublst in lst]

Demo:

>>> lst = [[['A'], ['B'], ['C']], [['A'], ['B'], ['D'], ['E']], [['B'], ['C']]]
>>> [[item for inner in sublst for item in inner] for sublst in lst]
[['A', 'B', 'C'], ['A', 'B', 'D', 'E'], ['B', 'C']]
>>> [[inner[0] for inner in sublst] for sublst in lst]
[['A', 'B', 'C'], ['A', 'B', 'D', 'E'], ['B', 'C']]

Upvotes: 2

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