lambda

In Python, the lambda keyword creates anonymous functions, also known as lambda functions. These functions are defined using the lambda keyword followed by a list of parameters, a colon, and an expression. Python evaluates the expression and returns the result when you call the lambda function.

Python lambda Keyword Examples

Here’s a quick example of using the lambda keyword to provide the key function and sort a dictionary by values:

Python 
>>> students = {
...     "Alice": 89.5,
...     "Bob": 76.0,
...     "Charlie": 92.3,
...     "Diana": 84.7,
...     "Ethan": 88.9,
...     "Fiona": 95.6,
...     "George": 73.4,
...     "Hannah": 81.2,
... }

>>> dict(sorted(students.items(), key=lambda item: item[1]))
{
    'George': 73.4,
    'Bob': 76.0,
    'Hannah': 81.2,
    'Diana': 84.7,
    'Ethan': 88.9,
    'Alice': 89.5,
    'Charlie': 92.3,
    'Fiona': 95.6
}

In this example, you sort the dictionary by value in ascending order. To do this, you use a lambda function that takes a two-value tuple as an argument and returns the second item, which has an index of 1.

Python lambda Keyword Use Cases

  • Creating callables for operations that can be expressed in a single expression
  • Combining short anonymous functions with functions like map(), filter(), and reduce()
  • Performing small operations that are used only once or very locally within your code
How to Use Python lambda Functions

Tutorial

How to Use Python Lambda Functions

In this step-by-step tutorial, you'll learn about Python lambda functions. You'll see how they compare with regular functions and how you can use them in accordance with best practices.

intermediate best-practices python

For additional information on related topics, take a look at the following resources:

By Leodanis Pozo Ramos • Updated Jan. 6, 2025 • Reviewed by Dan Bader
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