Lambda Functions

A lambda function is a small, anonymous function defined using the lambda keyword instead of def. It can take any number of arguments but is limited to a single expression, which is automatically returned.

Lambda Syntax

square = lambda x: x * x
print(square(5))   # 25

This is functionally equivalent to writing a full function definition, just far more compact for simple, one-line logic.

def square(x):
    return x * x

Using Lambda with Built-In Functions

Lambda functions are most commonly used as short, throwaway functions passed into other functions like map(), filter(), and sorted().

numbers = [1, 2, 3, 4, 5]

# Double every number
doubled = list(map(lambda x: x * 2, numbers))

# Keep only even numbers
evens = list(filter(lambda x: x % 2 == 0, numbers))

# Sort a list of tuples by the second value
pairs = [(1, "b"), (2, "a")]
pairs.sort(key=lambda p: p[1])

Coming Up Next

Next, you'll get a tour of some of Python's most useful built-in functions that you'll rely on constantly while working with data.