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Python Program to Flatten a Nested List

Intermediate · Lists

A nested list is a list that contains other lists, such as [1, [2, 3], [4, [5]]]. Flattening it means pulling every value out into one plain list: [1, 2, 3, 4, 5]. This program uses a recursive function, so it works however deep the lists are nested.

Program

flatten_nested_list.py
def flatten(items):
flat = []
for item in items:
if isinstance(item, list):
flat.extend(flatten(item)) # flatten the inner list first
else:
flat.append(item)
return flat
nested = [1, [2, 3], [4, [5, 6, [7]]], 8, []]
print("Nested list:", nested)
print("Flat list:", flatten(nested))
Output
Nested list: [1, [2, 3], [4, [5, 6, [7]]], 8, []]
Flat list: [1, 2, 3, 4, 5, 6, 7, 8]

How it works

  1. isinstance(item, list) checks whether an element is itself a list.
  2. A plain value is appended to flat straight away.
  3. An inner list is handed back to flatten(), which returns it as a flat list, and extend() adds all of its values.
  4. Because the function calls itself for every inner list, a list nested three levels deep, like [5, 6, [7]], is handled the same way as one level.
  5. The empty list at the end adds nothing, so it simply disappears.

Other ways to write it

One level deep with a list comprehension

One level deep with a list comprehension
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
# Read it like two loops: for each row, for each item in the row
flat = [item for row in matrix for item in row]
print("Flat list:", flat)
Output
Flat list: [1, 2, 3, 4, 5, 6, 7, 8, 9]

For a list of lists, one level deep, a comprehension with two for parts is the usual one-liner. It does not reach lists nested any deeper.

Using itertools.chain()

Using itertools.chain()
from itertools import chain
nested = [[1, 2], [3, 4, 5], [6]]
flat = list(chain.from_iterable(nested))
print("Flat list:", flat)
Output
Flat list: [1, 2, 3, 4, 5, 6]

chain.from_iterable() reads the inner lists one after another, which is fast and clear for one level of nesting.

Try it yourself

Add a string such as "hi" to the nested list and check that it stays whole in the flat list.

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