Python Lambda — Full Notes

What is a Lambda Function?

Lambda functions are used when you need a short, throwaway function.

Lambda Syntax

lambda arguments : expression

The expression is evaluated and returned automatically.

Basic Lambda Examples

Add 10 to a number

x = lambda a : a + 10
print(x(5))

Multiply two numbers

x = lambda a, b : a * b
print(x(5, 6))

Sum three numbers

x = lambda a, b, c : a + b + c
print(x(5, 6, 2))

Why Use Lambda Functions?

Lambda functions are extremely useful with closures and function factories.

Function returning a lambda

def myfunc(n):
  return lambda a : a * n

mydoubler = myfunc(2)
print(mydoubler(11))

Triple any number

def myfunc(n):
  return lambda a : a * n

mytripler = myfunc(3)
print(mytripler(11))

Doubler + Tripler together

def myfunc(n):
  return lambda a : a * n

mydoubler = myfunc(2)
mytripler = myfunc(3)

print(mytripler(11))

Lambda with Built-in Functions

Lambda works perfectly with map(), filter(), and sorted().

Using lambda with map()

numbers = [1, 2, 3, 4, 5]
doubled = list(map(lambda x: x * 2, numbers))
print(doubled)

Using lambda with filter()

numbers = [1, 2, 3, 4, 5, 6, 7, 8]
odd_numbers = list(filter(lambda x: x % 2 != 0, numbers))
print(odd_numbers)

Using lambda with sorted()

students = [("Emil", 25), ("Tobias", 22), ("Linus", 28)]
sorted_students = sorted(students, key=lambda x: x[1])
print(sorted_students)

Sort strings by length

words = ["apple", "pie", "banana", "cherry"]
sorted_words = sorted(words, key=lambda x: len(x))
print(sorted_words)

When to Use Lambda?

Use lambda when you need a small, quick function inside map(), filter(), sorted(), or as a return value from functions.