meet it

A function is a value you can pass around.

Write a function's name without brackets and you get the function itself, not its answer. You can store it in a variable, put it in a dictionary, or hand it to another function to call later.

sorted, max and min take a . They call it on every item and compare the answers, so sorted(words, key=len) puts the shortest words first.

For a one-off key, write a : lambda film: film[1] is a tiny function with no name that returns its one expression. Keep lambdas short. Anything bigger deserves a proper def.

films = [("Up", 2009), ("Jaws", 1975), ("Coco", 2017)]
oldest = min(films, key=lambda film: film[1])
by_title = sorted(films, key=lambda film: film[0])
print(oldest)
print(by_title)
shout = str.upper
print(shout("encore"))

key=len hands over the function, while key=len() would try to call it straight away.

guess first

A function that calls a function. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

def double(n):
    return n * 2

def twice(action, value):
    return action(action(value))

print(twice(double, 5))
print(twice(lambda word: word + "!", "hi"))

your turn

Your turn. Rank the players by score.

Each item is a pair like ("Ana", 42). Fill the blank so the lambda returns the score part of the pair. reverse=True puts the biggest first.

Your code

press run to see what happens

fix it

Shortest names first, but it crashes. Fix it!

Broken code to fix

press run to see what happens

The crash happens on line 2, before sorted even starts. What do the brackets after len do?
Brackets call len straight away, with nothing to measure. sorted wants the function itself.
Remove the brackets so it says key=len, then run.

nailed it

you passed functions around like values. :D

A name without brackets is the function itself.
key= tells sorted, max and min what to compare.
A lambda is a tiny one-line function with no name.

new words in your collection

key function
lambda

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