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Module 034: Lambda Functions

Phase: 4. Functions | Estimated time: 1.5 hours | Milestone Project: No

Prerequisites

  • Module 031 (Functions: Basics)
  • Module 032 (Function Arguments)

Learning Objectives

By the end of this module, you will be able to:

  • Write lambda functions with the lambda keyword
  • Know when lambdas are appropriate (short callbacks)
  • Understand the limitations of lambdas (single expression only)
  • Use lambdas with sorted() and key=
  • Use lambdas with map() and filter() briefly

Why This Matters

Lambdas allow you to write small, throwaway functions inline without a def statement. They are essential for callbacks, sorting with custom keys, and functional programming patterns.

Concept Explanation

Lambda Syntax

lambda arguments: expression

A lambda is a function without a name that evaluates a single expression:

square = lambda x: x ** 2
print(square(5))  # 25

This is equivalent to:

def square(x):
    return x ** 2

When to Use Lambdas

Use lambdas for short, simple operations passed directly as arguments:

  • Sorting with custom keys
  • Callbacks for GUI/event handling
  • Short transformations with map() and filter()

Limitations

  • Single expression only — no statements, no assignments, no loops.
  • No docstrings — hard to self-document.
  • Harder to debug — no name in tracebacks.

Sorting with key=lambda

students = [("Alice", 85), ("Bob", 72), ("Charlie", 90)]
students.sort(key=lambda s: s[1])  # sort by grade
print(students)  # [('Bob', 72), ('Alice', 85), ('Charlie', 90)]

Lambda with map() and filter()

nums = [1, 2, 3, 4, 5]
doubled = list(map(lambda x: x * 2, nums))
evens = list(filter(lambda x: x % 2 == 0, nums))

Lambda with Conditional Expression

max_val = lambda a, b: a if a > b else b
print(max_val(10, 20))  # 20

Common Pitfalls

  1. Trying to use statements: lambda x: return x + 1 is invalid — return is a statement.
  2. Overusing lambdas: If it's more than one expression, use def.
  3. Capturing loop variables: Lambdas in loops capture by reference, not by value.

Hands-On Walkthrough

  1. Write a lambda that returns the absolute value and test it.
  2. Sort a list of strings by their length using sorted() with key=lambda.
  3. Use map() with a lambda to convert temperatures from Celsius to Fahrenheit.
  4. Use filter() with a lambda to keep only positive numbers from a list.

Key Takeaways

  • Lambda: lambda args: expression — anonymous, inline, single expression.
  • Best for short callbacks (sorting, mapping, filtering).
  • Cannot contain statements (no return, if/else blocks, loops).
  • If it's complex, use def instead.

Further Reading

Next Module

Continue to Module 035: Recursion.