Python lambda functions

A lambda function in Python is a small anonymous function defined using the lambda keyword. Unlike regular functions defined with the def keyword, lambda functions are typically used for simple, short operations and are often used in places where functions are required as arguments.

Characteristics of Lambda Functions

  • Anonymous: They do not have a name.
  • Single expression: They contain only a single expression and return its result.
  • Syntax: They are defined using the lambda keyword followed by the arguments, a colon, and the expression.

Syntax

lambda arguments: expression

Examples

1. Basic Usage:

A simple lambda function that adds 10 to a given number:

add_ten = lambda x: x + 10
print(add_ten(5))  # Output: 15

2. Multiple Arguments:

A lambda function that multiplies two numbers:

multiply = lambda x, y: x * y
print(multiply(3, 4))  # Output: 12

3. Using Lambda with map():

Applying a lambda function to each element in a list using map:

numbers = [1, 2, 3, 4, 5]
squared = map(lambda x: x ** 2, numbers)
print(list(squared))  # Output: [1, 4, 9, 16, 25]

4. Using Lambda with filter():

Filtering a list to get only even numbers using filter:

numbers = [1, 2, 3, 4, 5, 6]
even_numbers = filter(lambda x: x % 2 == 0, numbers)
print(list(even_numbers))  # Output: [2, 4, 6]

5. Using Lambda with sorted():

Sorting a list of tuples by the second element:

points = [(1, 2), (3, 1), (5, -1), (2, 3)]
sorted_points = sorted(points, key=lambda x: x[1])
print(sorted_points)  # Output: [(5, -1), (3, 1), (1, 2), (2, 3)]

Here:

  • Basic Usage: The lambda function lambda x: x + 10 takes one argument x and returns x + 10.
  • Multiple Arguments: The lambda function lambda x, y: x * y takes two arguments x and y and returns their product.
  • Using Lambda with map(): The map function applies the lambda function lambda x: x ** 2 to each element in the numbers list, resulting in a list of squared numbers.
  • Using Lambda with filter(): The filter function applies the lambda function lambda x: x % 2 == 0 to each element in the numbers list, returning only the even numbers.
  • Using Lambda with sorted(): The sorted function sorts the list of tuples points based on the second element of each tuple, as specified by the lambda function lambda x: x[1].


Lambda functions are particularly useful for short operations that are used only once or are passed as arguments to higher-order functions like map(), filter(), and sorted().