ይህ ትምህርት ገና ወደ አማርኛ አልተተረጎመም፤ ስለዚህ በእንግሊዝኛ ቀርቧል። የእንግሊዝኛውን ገጽ ክፈቱ

Functions are values

3 min read

In lesson 4.2 you sorted words with key=str.lower and were promised an explanation. Here it is: a function is a value, like a number or a list, so you can give it a second name or hand it to another function. By the end you'll sort and pick from any list by any rule you choose.

A function is a value

Predict what this prints.

ውጤቱን ገምቱ

Decide before you look. Guessing wrong is how this sticks.

def greet(name):
    print(f"Selam, {name}!")

say = greet
say("Abel")

Python prints

Selam, Abel!

say = greet gives the same function a second name, just like b = a gave a list a second name in lesson 4.8. Calling say calls that function.

def makes a function object and gives it a name. Assignment can give it another. Step through it: two names, one function.

Two names, one function

Press Next to run one line at a time. Each arrow shows the object a name refers to. Each call gets its own frame of names, below the frame that called it.Each row is one step: the line that just ran, and the object each name refers to after it, frame by frame, oldest first.

1def greet(name):
2    print(f"Selam, {name}!")
3 
4say = greet
5say("Abel")

Step 1 of 5

Every step: the line that ran, and the object each name refers to after it
Step	Line	Names and objects
1	1	global greet → F1 function greet
2	4	global greet → F1 function greet say → F1 function greet
3	5	global greet → F1 function greet say → F1 function greet greet(name) name → S2 'Abel'
4	2	global greet → F1 function greet say → F1 function greet greet(name) name → S2 'Abel' greet(name) returns None to global.
5	5	global greet → F1 function greet say → F1 function greet S2 'Abel' (no name refers to it now)

The frame is labeled greet, not say: the call runs the function object, whatever name you reached it by. Because functions can be named, passed, and returned like any other value, Python has first-class functionsfirst-class functionsA function that is a value like any other: it can be given names, put in a list, passed to a function, and returned..

Passing a function to sorted

sorted can take a function as its key. It calls that function on each item and sorts by the results, while keeping the items themselves. The three rounds of a coffee ceremony, sorted by the length of their names:

ውጤቱን ገምቱ

Decide before you look. Guessing wrong is how this sticks.

rounds = ["bereka", "tona", "abol"]
print(sorted(rounds, key=len))

Python prints

['tona', 'abol', 'bereka']

len gives 6, 4, and 4, so the two 4-letter words come first. They tie, and sorted keeps tied items in the order they had: tona was before abol. The key replaces alphabetical order entirely.

A key functionkey functionA function you pass as key= to sorted, min, or max. It's called on each item, and the results decide the order. takes one item and returns what to compare. key=len compares lengths; key=str.lower compares lowercase versions, so capitals no longer sort first. Items with equal keys keep their original order. Pass the function itself, not a call:

የተለመደ ስህተት

This breaks

rounds = ["bereka", "tona", "abol"]
print(sorted(rounds, key=len()))

Python says

TypeError: len() takes exactly one argument (0 given)

The fix

rounds = ["bereka", "tona", "abol"]
print(sorted(rounds, key=len))

len() calls len right away, with nothing to measure. key=len hands sorted the function, and sorted calls it on each word.

ጥያቄ

In sorted(words, key=len), what is len?

Your own key, and lambda

Any function that takes one item works as a key. Here each item is an (item, price) tuple, and the key picks out the price:

prices = [("coffee", 40), ("bread", 25), ("tea", 15)]

def price_of(pair):
    return pair[1]

print(sorted(prices, key=price_of))
print(max(prices, key=price_of))

Ctrl or Cmd + Enter runs

max and min take a key too: max returns the whole tuple with the biggest price, not just 40. For a key this small, writing a whole def feels heavy. A lambdalambdaA small function written in one expression, without def or a name: lambda pair: pair[1]. is a one-line function with no name: lambda, the parameter, a colon, and the value to return.

prices = [("coffee", 40), ("bread", 25), ("tea", 15)]
print(min(prices, key=lambda pair: pair[1]))

Ctrl or Cmd + Enter runs

lambda pair: pair[1] does exactly what price_of does. Use a lambda for a short key used once. Use a named function when the rule takes more than one line, when you use it in several places, or when a name like price_of explains it better than pair[1] does.

መልመጃ

Contacts in name order

Some contacts were typed in lowercase, so plain sorted puts Hana and Selam first. Print the names in alphabetical order, ignoring case, with each number.

Output

abel: 0900 000 001
dawit: 0900 000 004
Hana: 0900 000 002
Selam: 0900 000 003

Your code

contacts = {"Selam": "0900 000 003", "abel": "0900 000 001", "Hana": "0900 000 002", "dawit": "0900 000 004"}
for name in sorted(contacts):
    print(f"{name}: {contacts[name]}")

Ctrl or Cmd + Enter runs

መፍትሄውን አሳይ

This is one way to solve it, not the only one. If yours prints the same thing, it works.

contacts = {"Selam": "0900 000 003", "abel": "0900 000 001", "Hana": "0900 000 002", "dawit": "0900 000 004"}
for name in sorted(contacts, key=str.lower):
    print(f"{name}: {contacts[name]}")

ዋና ዋና ነጥቦች

A function is a value: say = greet gives one function two names.
key= takes a function, not a call: key=len, never key=len().
sorted, min, and max call the key on each item and compare the results; ties keep their order.
lambda pair: pair[1] is a one-line function with no name, good for short keys.
Prefer a named function when the rule is long, reused, or needs a name to be clear.