A Beginner‘s Guide to Flattening Lists in Python

Welcome friend! Today I will be your guide through the world of flattening lists in Python. Whether you are just getting started with Python or are looking to expand your data wrangling skills, this comprehensive guide has everything you need to flatten nested lists with ease.

What Exactly is "Flattening" Anyway?

In programming terms, flattening refers to converting a multi-dimensional data structure into a simpler one-dimensional form. For example, transforming this:

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

Into this:

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

We took a list of lists and "flattened" it into a single flat list containing all the data.

Why would we want to do this?

Complex nested structures may come from loading JSON configuration files, fetching database rows, or during intermediate data processing pipelines. Many Python libraries expect simple flat lists or arrays during usage.

Flattening them makes it much easier to:

  • Iterate through all values
  • Pass data into models, statistics functions, etc
  • Understand and visualize the data
  • Simplify further data analysis/munging

So in short – flattening removes unnecessary complexity, allowing you to focus on accessing the data itself.

This guide will demonstrate both basic and advanced techniques for flattening in Python through easy-to-follow examples. Let‘s get started!

Flattening Lists with Loops

The most straightforward approach uses nested for loops to iterate through the list of lists. Here is a simple example:

nested_list = [[1, 2], [3, 4], [5, 6]]
flat_list = []

for inner_list in nested_list:
  for item in inner_list:

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


  1. Initialize a nested list and empty flat list
  2. Use outer loop to access each inner list
  3. Inner loop goes through current inner list
  4. Append elements from inner list to flat list

The end result is our simple 1D flattened output!

While this showcases basic Python, it works quite well for small datasets. But performance degrades with larger lists due to the explicit Python loops and repeatedly appending to flat_list.

We can optimize this slightly using the list extend() method:

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

flat_list = []
for inner_list in nested_list:

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

extend() appends an entire iterable onto the caller list, reducing overall operations.

So in summary, looping provides a simple and readable approach to basic list flattening in Python. But we need more advanced techniques for larger datasets.

Leveraging itertools.chain for Speed

The built-in itertools module contains many useful methods for wrangling with iterators and generators in Python.

One handy function is chain.from_iterable – this chains together multiple separate iterables into a single stream of data.

In other words, it flattens iterables!

Let‘s see it action:

from itertools import chain

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

flat_iterator = chain.from_iterable(nested_list)

flat_list = list(flat_iterator) # convert to list
print(flat_list) # [1, 2, 3, 4, 5, 6]

By converting the result into a regular list, we can access the final flattened data.

The key benefits of using chain are:

  • Avoids creating intermediate list structures
  • Generators use lazy evaluation (only as needed)
  • Much faster flattening for large datasets
  • Simpler code than manual looping

So itertools.chain is ideal when working with huge lists or performance-critical applications.

Recursion to Flatten Irregular Structures

Our previous examples work great for uniform nested lists. But what about more irregular data structures with varied depths?

For example:

messy_list = [1, 2, [3, 4, [5, 6]], [[7, 8], 9], 10]

This has 4 levels of nesting including single items, lists, lists within lists.

Trying to use loops or chaining would be very tricky and messy with this input!

Instead, we can elegantly flatten any data shape with…recursion!

The key intuition behind recursion is:

  • Base Case – Is element a simple value? If yes, append to output list.
  • Recursive Case – Is element another nested list? If yes, call function recursively on that sub-list.

This allows flattening to arbitrary depths in one clean sweep, no matter how irregular the structure.

Let‘s see an implementation:

def flatten(nested_list):

    flat_list= []

    for element in nested_list:
        if isinstance(element, list):
            flatten(element) # recursive call

    return flat_list

messy_list = [1, 2, [3, 4, [5, 6]], [[7, 8], 9], 10]  

print(flatten(messy_list)) # [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]


  1. We check if current element is a list
  2. If yes, recursively call flatten on that sub-list to dive deeper
  3. The base case is appending a simple value to the output list
  4. Finally we unwind back up the call stack to build the full flattened list

So in summary, recursion provides an elegant and flexible way to flatten even highly irregular data structures in Python.

Miscellaneous Flattening Approaches

There are a couple more flattening alternatives worth mentioning:

Converting to set and back:

from itertools import chain 

flat_set = set(chain.from_iterable(nested_list))
flat_list = list(flat_set) 

This approach implicitly removes any duplicate values.

Using NumPy:

import numpy as np

flat_array = np.array(nested_list).ravel()
flat_list = flat_array.tolist()

NumPy has vectorized operations to flatten arrays in just one step.

List comprehensions:

flat_list = [x for inner_list in nested_list 
                 for x in inner_list]

Comprehensions provide a concise way to flatten and transform lists.

Guidelines for Choosing the Best Method

We‘ve covered several approaches, each with trade-offs. But which one should you actually use?

Here are some general guidelines:

  • For small datasets, use simple loops. More readable.
  • Handling large data? Use itertools.chain for speed.
  • Dealing with irregular nested structures? Choose recursion for flexibility.
  • Other methods like NumPy/comprehensions useful for niche cases.

Also consider factors like your team‘s experience level, ease of maintenance, and debugging needs when selecting a technique.


We covered a ton of material on flattening in Python including:

  • Loops to iterate through list data
  • itertools.chain for faster flattening
  • Recursion to handle irregular structures
  • And niche approaches like NumPy/ comprehensions

You are now equipped to handle any type of nested list flattening across basic to advanced scenarios.

I hope you found this guide useful! Please reach out if you have any other questions – I‘m always happy to chat more about effective data wrangling.

Happy coding!