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

Tuples and unpacking

4 min read

Some data should never change. A coffee ceremony always has the same three rounds, in the same order. In this lesson you'll store fixed data like that in a tuple, give its parts names in one line, and finally see how the swap shortcut from lesson 1.1 works.

A list that can't change

A tupletupleAn ordered group of values that can't be changed after it's made, written with round brackets: ("abol", "tona", "bereka"). is written with round brackets instead of square ones. You read it exactly like a list: indexes, slices, len(), for, and in all work.

rounds = ("abol", "tona", "bereka")
print(rounds[1])
print(len(rounds))
for name in rounds:
    print(name)

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The difference is that a tuple can never change. The ceremony has exactly these three rounds, so a tuple says what the data is: fixed. It also stops anyone, including you later, from changing it by accident:

የተለመደ ስህተት

This breaks

rounds = ("abol", "tona", "bereka")
rounds[0] = "buna"

Python says

TypeError: 'tuple' object does not support item assignment

The fix

rounds = ("abol", "tona", "bereka")
first = rounds[0]

It's the same message strings give: a tuple doesn't allow something[position] = value. Tuples have no append or remove either. If the data needs to change, it should be a list.

You've already seen a tuple: lesson 2.3 promised a shorter way to write answer == "yes" or answer == "y". Here it is:

answer = input("Ready? ")
if answer in ("yes", "y"):
    print("Let's go!")

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Input

One answer per line, given to input() in order.

in asks whether a value is one of the items. Add "Y" to the tuple so a capital Y works too.

The comma makes the tuple

Round brackets already mean something in Python: (2 + 3) * 4 groups a calculation. So what is (5)? Predict.

ውጤቱን ገምቱ

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

print(type((5)))
print(type((5,)))

Python prints

<class 'int'>
<class 'tuple'>

(5) is just 5 in grouping brackets, so it's an int. The comma is what makes a tuple: (5,) is a tuple with one item.

A tuple with one item needs a trailing comma: ("abol",). With two or more items the commas are already there.

Unpacking: a name for each part

A tuple often holds the parts of one thing, like an item and its price. You can give each part its own name in one line. This is called unpackingunpackingGiving each item of a tuple or list its own name in one line: item, price = order.:

order = ("macchiato", 35)
item, price = order
print(item)
print(price)

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The first name gets the first item, the second name the second. The number of names must match the number of items:

የተለመደ ስህተት

This breaks

order = ("macchiato", 35, "large")
item, price = order

Python says

ValueError: too many values to unpack (expected 2, got 3)

The fix

order = ("macchiato", 35, "large")
item, price, size = order

The message counts for you: two names ("expected 2"), three items ("got 3"). Add a name for each item.

Unpacking also works in a for loop. When each item of a list is a tuple, give the parts names right in the loop:

orders = [("macchiato", 35), ("sambusa", 20)]
for item, price in orders:
    print(item, "costs", price)

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The swap, explained

In lesson 1.1 you swapped two values with a third name, temp, and we said the shortcut a, b = b, a would make sense in Module 4. Now it does. Step through it:

Step through it

Press Next to run one line at a time and watch each name change.Each row is one step: the line that just ran, and every name's value after it.

1a = "tea"
2b = "coffee"
3a, b = b, a
4print(a, b)

Names and their values after line 1
Name	Value after line 1
a	'tea'new
b	not made yet

Output so far

coffee tea

Step 1 of 4

Every step: the line that ran and each name's value after it
Step	Line	a	b	Printed
1	1	'tea'	not made yet
2	2	'tea'	'coffee'
3	3	'coffee'	'tea'
4	4	'coffee'	'tea'	coffee tea

Python works out the right side first, as always. b, a is a tuple: ("coffee", "tea"). Only then does it unpack that tuple into a and b. The tuple already referred to both old values before either name moved, so nothing is lost.

The rest of the items

A star before a name collects all the items that are left, as a list. Predict this:

ውጤቱን ገምቱ

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

first, *rest = [90, 75, 60, 45]
print(first)
print(rest)

Python prints

90
[75, 60, 45]

first gets the first item. *rest gets everything left over, and it's always a list, even when the items came from a tuple.

The star can go on any one name: *others, last = scores puts the last item in last. Try it with the scores above.

ጥያቄ

After name, price = ("shai", 15), what is price?

መልመጃ

A café receipt

Each item of order is a tuple of an item and its price. Print a receipt: each item in a column 12 wide on the left, its price 8 wide on the right with two decimals (lesson 1.4), then the total.

Output

Macchiato      35.00
Sambusa        20.00
Water          15.00
Total          70.00

Your code

order = [("Macchiato", 35), ("Sambusa", 20), ("Water", 15)]
# Unpack each tuple in the loop, print a line, and keep a running total

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መፍትሄውን አሳይ

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

order = [("Macchiato", 35), ("Sambusa", 20), ("Water", 15)]
total = 0
for item, price in order:
    print(f"{item:<12}{price:>8.2f}")
    total = total + price
print(f"{'Total':<12}{total:>8.2f}")

ዋና ዋና ነጥቦች

A tuple is like a list that can't change: ("abol", "tona", "bereka"). Use it for fixed data.
The comma makes a tuple: (5) is an int, (5,) is a tuple.
Unpacking gives each item a name: item, price = order. The counts must match.
a, b = b, a builds the tuple on the right first, then unpacks it, so nothing is lost.
first, *rest = items puts the leftover items in a list.