071. Lambda Functions

The RAM manager shows every process — but the list is unsorted. He wants the top memory hogs at the top.

He tries to sort the process list by RSS memory. A def comparator seems like overkill for one line. A classmate shows him lambda:

processes = [
    {"name": "chrome",  "rss_mb": 420},
    {"name": "python",  "rss_mb": 85},
    {"name": "slack",   "rss_mb": 310},
]

# Sort largest-first by RSS
top = sorted(processes, key=lambda p: p["rss_mb"], reverse=True)
# → chrome (420), slack (310), python (85)

He also adds a quick MB-to-GB converter for the summary line:

mb_to_gb = lambda mb: mb / 1024

print(f"Total: {mb_to_gb(2048):.1f} GB")  # → Total: 2.0 GB

💡 Rule of thumb: Use lambda when the function fits in one expression and you only need it once (as a key=, callback, or one-liner transform). Use def for anything longer, recursive, or reused.

Learning objectives

Create lambda functions for simple transformations
Use lambda as key= argument in sorted()
Recognize when lambda is appropriate vs def

Key concepts

lambda
anonymous function
higher-order functions

Try it

Concept detail

lambda creates a small anonymous function in a single expression.

lambda x: x * 2          # equivalent to: def f(x): return x * 2
lambda a, b: a + b        # two parameters
lambda p: p["score"]      # key function for sorted/min/max

Syntax: lambda :

Common uses:

key= in sorted(), min(), max()
Quick one-off transformations passed as arguments
Simple callbacks

Lambdas cannot contain:

Statements (if/else blocks, loops, assignments — use def)
Multiple expressions

WHY lambdas exist: passing short functions as arguments without the verbosity of a full def. sorted(items, key=lambda x: x[“score”]) reads like a sentence.

Use lambdas sparingly — a named def is clearer for anything non-trivial or reused.

Solution

double = lambda x: x * 2

to_celsius = lambda f: (f - 32) * 5 / 9

players = [
    {"name": "Alice", "score": 95},
    {"name": "Bob",   "score": 78},
    {"name": "Carol", "score": 88},
]
ranked = sorted(players, key=lambda p: p["score"], reverse=True)

Tests

def test_double():
    assert double(5) == 10
    assert double(3) == 6
    assert double(0) == 0

def test_to_celsius_freezing():
    assert abs(to_celsius(32) - 0.0) < 0.001

def test_to_celsius_boiling():
    assert abs(to_celsius(212) - 100.0) < 0.001

def test_to_celsius_body():
    assert abs(to_celsius(98.6) - 37.0) < 0.1

def test_ranked_first():
    assert ranked[0]["name"] == "Alice"

def test_ranked_last():
    assert ranked[-1]["name"] == "Bob"