Flatten a nested list in Python

To flatten a list of lists means to take the items out of the inner lists and put them all in one list, in the same order.

The program

nested = [[1, 2], [3, 4, 5], [], [6]]

flat = []
for inner in nested:
    for item in inner:
        flat.append(item)

print(flat)

[1, 2, 3, 4, 5, 6]

How it works

• The outer loop takes one inner list at a time. The inner loop takes each item from that list and appends it to flat.
• The empty list [] adds nothing, so it needs no special case.

The same two loops fit in one list comprehension. The for parts are in the same order as in the loop version:

nested = [[1, 2], [3, 4, 5], [], [6]]
flat = [item for inner in nested for item in inner]
print(flat)

[1, 2, 3, 4, 5, 6]

Lists nested to any depth

The versions above go only one level deep. When lists can hold lists that hold more lists, a recursive function handles every level: when an item is itself a list, the function calls itself on that item.

def flatten(items):
    result = []
    for item in items:
        if isinstance(item, list):
            result.extend(flatten(item))
        else:
            result.append(item)
    return result

print(flatten([1, [2, [3, [4]], 5], [[6]]]))

[1, 2, 3, 4, 5, 6]

result.extend(...) adds every item of a list to result, while append would add the list as one item.