Lambda Functions in Python: What Are They Good For?

By Dan Bader — Get free updates of new posts here.

An introduction to “lambda” expressions in Python: What they’re good for, when you should use them, and when it’s best to avoid them.

The lambda keyword in Python provides a shortcut for declaring small anonymous functions. Lambda functions behave just like regular functions declared with the def keyword. They can be used whenever function objects are required.

For example, this is how you’d define a simple lambda function carrying out an addition:

>>> add = lambda x, y: x + y
>>> add(5, 3)
8

You could declare the same add function with the def keyword:

>>> def add(x, y):
...     return x + y
>>> add(5, 3)
8

Now you might be wondering: Why the big fuss about lambdas? If they’re just a slightly more terse version of declaring functions with def, what’s the big deal?

Take a look at the following example and keep the words function expression in your head while you do that:

>>> (lambda x, y: x + y)(5, 3)
8

Okay, what happened here? I just used lambda to define an “add” function inline and then immediately called it with the arguments 5 and 3.

Conceptually the lambda expression lambda x, y: x + y is the same as declaring a function with def, just written inline. The difference is I didn’t bind it to a name like add before I used it. I simply stated the expression I wanted to compute and then immediately evaluated it by calling it like a regular function.

Before you move on, you might want to play with the previous code example a little to really let the meaning of it sink in. I still remember this took me a while to wrap my head around. So don’t worry about spending a few minutes in an interpreter session.

There’s another syntactic difference between lambdas and regular function definitions: Lambda functions are restricted to a single expression. This means a lambda function can’t use statements or annotations—not even a return statement.

How do you return values from lambdas then? Executing a lambda function evaluates its expression and then automatically returns its result. So there’s always an implicit return statement. That’s why some people refer to lambdas as single expression functions.

<strong><em>Improve Your Python</em></strong> with a fresh 🐍 <strong>Python Trick</strong> 💌 every couple of days

Improve Your Python with a fresh 🐍 Python Trick 💌 every couple of days

🔒 No spam ever. Unsubscribe any time.

This article was filed under: programming, and python.

Related Articles:

Python’s Functions Are First-Class – Python’s functions are first-class objects. You can assign them to variables, store them in data structures, pass them as arguments to other functions, and even return them as values from other functions.
Python Decorators: A Step-By-Step Introduction – Understanding decorators is a milestone for any serious Python programmer. Here’s your step-by-step guide to how decorators can help you become a more efficient and productive Python developer.
Using get() to return a default value from a Python dict – Python’s dictionaries have a “get” method to look up a key while providing a fallback value. This short screencast tutorial gives you a real-world example where this might come in handy.
A Python Riddle: The Craziest Dict Expression in the West – Let’s pry apart this slightly unintuitive Python dictionary expression to find out what’s going on in the uncharted depths of the Python interpreter.
Writing Clean Python With Namedtuples – Python comes with a specialized “namedtuple” container type that doesn’t seem to get the attention it deserves. It’s one of these amazing features in Python that’s hidden in plain sight.

Latest Articles:

Interfacing Python and C: The CFFI Module – How to use Python’s built-in CFFI module for interfacing Python with native libraries as an alternative to the “ctypes” approach.
Write More Pythonic Code by Applying the Things You Already Know – There’s a mistake I frequently make when I learn new things about Python… Here’s how you can avoid this pitfall and learn something about Python’s “enumerate()” function at the same time.
Working With File I/O in Python – Learn the basics of working with files in Python. How to read from files, how to write data to them, what file seeks are, and why files should be closed.
How to Reverse a String in Python – An overview of the three main ways to reverse a Python string: “slicing”, reverse iteration, and the classic in-place reversal algorithm. Also includes performance benchmarks.
Mastering Click: Writing Advanced Python Command-Line Apps – How to improve your existing Click Python CLIs with advanced features like sub-commands, user input, parameter types, contexts, and more.
Working with Random Numbers in Python – An overview for working with randomness in Python, using only functionality built into the standard library and CPython itself.

← Browse All Articles

🐍 Python Tricks 💌