How to Use Lambda Functions in Python (Anonymous Functions)

Learn how to use lambda functions in Python. Master anonymous functions, inline callbacks, and sorting list data with lambda keys.

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Explanation

In Python, functions are first-class citizens, meaning they can be assigned to variables, passed as arguments to other functions, and returned from functions. While standard functions are declared using the `def` keyword, Python also provides a shorthand syntax for creating anonymous inline functions: `lambda` functions.

A lambda function is defined with the `lambda` keyword, followed by a list of arguments, a colon, and a single expression. The syntax is `lambda arguments: expression`. Unlike `def` functions, lambdas do not require a `return` statement; the result of the expression is returned automatically. They are limited to a single logical line, making them ideal for simple, one-off operations.

Lambda functions are most commonly used as arguments to higher-order functions like `sorted()`, `map()`, and `filter()`. For example, you can pass a lambda function to `sorted()`'s `key` argument to define custom sorting criteria, or use them as lightweight callback functions inside graphical or event-driven applications.

Step-by-Step Implementation

1
Write the lambda keyword followed by your comma-separated parameter names.
2
Place a colon, followed by the single expression you want to evaluate and return.
3
Pass the lambda directly as a callback to functions like sorted(), map(), or filter().

Code Example

This script demonstrates constructing lightweight anonymous functions using lambda.

lambda_functions.py

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# 1. Simple lambda function assigned to a variable
double = lambda x: x * 2
print("Double of 5:", double(5))

# 2. Multi-argument lambda
add = lambda x, y: x + y
print("Sum of 3 + 4:", add(3, 4))

# 3. Using lambda as a sorting key (sorting by age)
people = [
    {"name": "Alice", "age": 28},
    {"name": "Bob", "age": 22},
    {"name": "Charlie", "age": 35}
]
people.sort(key=lambda person: person["age"])
print("Sorted by age:", people)

Terminal Output

Double of 5: 10
Sum of 3 + 4: 7
Sorted by age: [{'name': 'Bob', 'age': 22}, {'name': 'Alice', 'age': 28}, {'name': 'Charlie', 'age': 35}]

Frequently Asked Questions

Can a lambda function contain loops or multiple statements?

No, lambda functions in Python are strictly limited to a single expression. If you need complex logic, loops, or variable assignments, you must define a standard function using def.

Should I assign lambda functions to variables?

Generally no. PEP 8 guidelines suggest using a standard def statement instead of assigning a lambda to a variable, as standard functions provide better stack trace names.

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