How to Flatten a 2D List in Python
Suppose we have a list of lists, or 2D list, in Python.
lst_of_lsts = [[1, 2, 3], [4, 5, 6], [7, 8]]
We want to flatten this list, or convert it into a 1D list.
flattened_lst = [1, 2, 3, 4, 5, 6, 7, 8]
We can use two simple for loops to obtain the flattened list.
flattened_lst =  for sublist in lst_of_lsts: for elem in sublist: flattened_lst.append(elem)
This is a very intuitive approach that gives us the correct flattened list.
Using List Comprehension
We can simplify the syntax a bit using list comprehension.
flattened_lst = [elem for sublist in lst_of_lsts for elem in sublist]
This is essentially the same as the for loop method above.
We can use
reduce() to achieve the same output.
reduce(func, iter) will take in 2 parameters: a function
func and an iterable
func(a, b) will take two parameters and perform some operation on them. In our case, it will be
operator.concat, which will concatenate, or combine, each element in
iter while following the reducer/accumulator pattern.
iter is simply an iterable object (list, tuple, dictionary, etc).
import functools flattened_lst = functools.reduce(operator.concat, lst_of_lsts)
Lastly, we can use the
import itertools flattened_lst = list(itertools.chain.from_iterable(lst_of_lsts))
from_iterable() function essentially runs a double for loop like in our first method.
def from_iterable(iterables): for it in iterables: for element in it: yield element
However, it returns an
itertools.chain object that we could use to iterate through every value in our list of lists.
In order to get the flattened list, we can convert this object to a single list using the
list() type conversion.
chain() and the Star Operator
The single star operator
* unpacks the sequence or collection into positional arguments. We can do something like this:
def sum(a, b): return a + b nums = (1, 2) res = sum(*nums)
With this in mind, we can pass in these positional arguments into the
chain() function from the previous method.
import itertools flattened_lst = list(itertools.chain(*lst_of_lsts))
More Python Articles
- How to Start an HTTP Server in Python
- How to Get All Keys with the Highest Value in Python
- How to Check if a Tuple Exists in a List in Python
- How to Sort a List of Dictionaries By Field in Python
- How to Sort a Dictionary by Value in Python
- How to Sort a List of Tuples Based on Multiple Elements
- How to Remove Duplicates from a List in Python
- How to Set Multiple Values of a List in Python
- How to Remove the Last N Elements of a List in Python
- How to Get the ASCII Value of a Character in Python
- How to Loop Over a String in Reverse in Python
- How to Create a Two Dimensional List in Python
- How to Migrate Data from MongoDB to Elasticsearch in Python
- How to Add Key-Value to Dictionary During List Comprehension in Python
- How to Fix "datetime is not JSON serializable" TypeError in Python
- How to Remove a Key From a Dictionary in Python
- Encoding and Decoding Bytes Explained
- How to Paginate/Scroll Elasticsearch Data using Python
- How to Print Without Newline in Python 3
- How to Get the Key with the Maximum Value in Python
- List Comprehension in Python Explained Visually
- How to Check if a String Contains Another String in Python
- What is Enumerate in Python?
- What is the Difference Between List and Tuple in Python?