Functions as Values, and Lambdas

Why this matters

In Python a function is a value, just like a number or a string. You can store it in a variable, put it in a list, and pass it to another function. That is how sorted(words, key=len) works: you handed sorted the function len and it calls it for you. This idea makes code flexible and short.

The idea

def shout(text):
    return text.upper() + "!"

say = shout            # no parentheses: this does not call it, it just gives it a second name
print(say("hi"))       # HI!

def apply_twice(func, value):
    return func(func(value))

print(apply_twice(shout, "a"))   # A!!

shout (a value) is different from shout("a") (a call that runs the function).

A lambda is a tiny function written inline when you only need it once:

double = lambda n: n * 2         # same as def double(n): return n * 2
people = [("Ada", 36), ("Bo", 29)]
youngest_first = sorted(people, key=lambda person: person[1])

A lambda has parameters, a colon, and one expression whose value is returned automatically. If you need more than one expression, write a normal def.

sorted, min, max and .sort all accept a key= function; that is the most common use of lambdas.

Try it

Use max(["pear", "fig", "banana"], key=len) and then try a lambda that picks the word with the last letter that comes latest in the alphabet.

Your turn

Write three functions:

• apply_twice(func, value): returns func(func(value)). For example apply_twice(lambda n: n + 3, 10) is 16.
• sort_people(people): people is a list of (name, age) tuples; return a new list sorted with the oldest first.
• longest(words): the longest word in a list (the first one if there is a tie).

Recap

• Functions are values: f is the function, f() calls it.
• A function can take another function as an argument (a “callback” or key).
• lambda args: expression is a one-line anonymous function.

Common mistakes

• Adding parentheses by accident: sorted(words, key=len()) calls len with no argument (TypeError). Pass len.
• Cramming logic into a lambda. If it needs more than a short expression, use def and give it a name.
• Forgetting key takes a function, not a value: key=3 fails.