Session 8 · Tuples: the magic photo album#

📖 Based on the blog posts The Magic Photo Album: Python Tuple and Python Task: Tuple.

The big idea#

A tuple is like a list that’s sealed: ordered, but it can’t be changed after it’s created. Use a tuple for a fixed group of values that belong together: a date, a GPS point, one row of data.

Everyday example: a printed photo. Once it’s taken, the place, date and moment are fixed. You can look at it, copy it and put it in an album, but you can’t edit the photo itself.

A tuple is ordered like a list but immutable: you can read, unpack, join and repeat tuples, but you can't change an item in place.

Morning: creating snapshots#

Round brackets, or just commas:

ooty = ("Ooty", "2024-07-01", "Visited Botanical Garden")
munnar = ("Munnar", "2023-12-15", "Saw Nilgiri Tahr in Eravikulam national park")
manjolai = ("Manjolai", "2023-05-21", "Enjoyed hill stations")
photo_album = [ooty, munnar, manjolai]      # a list of tuples
print(type(ooty), len(photo_album))

<class 'tuple'> 3

Noon and afternoon: reading and unpacking#

ooty = ("Ooty", "2024-07-01", "Visited Botanical Garden")
munnar = ("Munnar", "2023-12-15", "Saw Nilgiri Tahr in Eravikulam national park")

print(ooty[0], ooty[-1])                     # indexing works like lists
location, date, description = munnar         # unpacking
print(f"Location: {location}, Date: {date}")

Ooty Visited Botanical Garden
Location: Munnar, Date: 2023-12-15

Trying to edit a photo#

ooty = ("Ooty", "2024-07-01", "Visited Botanical Garden")
try:
    ooty[2] = "Visited Doddabeta Peak"
except TypeError as e:
    print("TypeError:", e)

TypeError: 'tuple' object does not support item assignment

To “change” a tuple, build a new one, for example via a list:

ooty = ("Ooty", "2024-07-01", "Visited Botanical Garden")
as_list = list(ooty)
as_list[2] = "Visited Doddabeta Peak"
ooty = tuple(as_list)            # a NEW tuple, bound to the same name
print(ooty)

('Ooty', '2024-07-01', 'Visited Doddabeta Peak')

Evening and night: joining, repeating, checking#

ooty = ("Ooty", "2024-07-01")
munnar = ("Munnar", "2023-12-15")
album = [ooty, munnar]

print(ooty + munnar)          # concatenate: a new, longer tuple
print(ooty * 2)               # repeat
print(ooty in album)          # membership
print(len(album))

('Ooty', '2024-07-01', 'Munnar', '2023-12-15')
('Ooty', '2024-07-01', 'Ooty', '2024-07-01')
True
2

Why use tuples?#

Safety: values that must not change can’t be changed by accident.
Dictionary keys: tuples can be keys ({(0, 0): "origin"}); lists can’t.
Returning several values from a function: return min_v, max_v returns a tuple.
Slightly faster and smaller than lists.

Common mistakes#

One-item tuple without a comma: ("x") is just the string "x". Write ("x",).
```
print(type(("x")), type(("x",)))
```
```
<class 'str'> <class 'tuple'>
```
Unpacking the wrong number: a, b = (1, 2, 3) raises ValueError: too many values to unpack.
“Immutable” doesn’t mean “frozen all the way down”: a tuple holding a list can’t swap the list for another, but the list inside can still change: t = (1, [2]); t[1].append(3) works.

Hands-on exercises#

The 19 tasks from Python Task: Tuple.

Task 1. Create a tuple of three fruits; print it and its type.

Solution

fruits = ("mango", "banana", "jackfruit")
print(fruits, type(fruits))

Task 2. Print the second element of t = ("apple", "banana", "cherry").

Solution

t = ("apple", "banana", "cherry")
print(t[1])     # banana

Task 3. Unpack t = (1, 2, 3) into a, b, c.

Solution

t = (1, 2, 3)
a, b, c = t
print(a, b, c)

Task 4. Concatenate t1 = (1, 2) and t2 = (3, 4).

Solution

t1, t2 = (1, 2), (3, 4)
print(t1 + t2)     # (1, 2, 3, 4)

Task 5. Repeat t = ("repeat",) three times.

Solution

t = ("repeat",)
print(t * 3)       # ('repeat', 'repeat', 'repeat')

Task 6. Count the 2s in t = (1, 2, 3, 2, 2, 4).

Solution

t = (1, 2, 3, 2, 2, 4)
print(t.count(2))  # 3

Task 7. Find the index of "c" in t = ("a", "b", "c", "d").

Solution

t = ("a", "b", "c", "d")
print(t.index("c"))   # 2

Task 8. Check whether 5 is in t = (1, 2, 3, 4).

Solution

t = (1, 2, 3, 4)
print(5 in t)      # False

Task 9. Print the length of t = ("one", "two", "three").

Solution

t = ("one", "two", "three")
print(len(t))      # 3

Task 10. From t = (0, 1, 2, 3, 4, 5), slice out the elements at indexes 2 to 4.

Solution

t = (0, 1, 2, 3, 4, 5)
print(t[2:5])      # (2, 3, 4): stop is excluded, so 5 not 4

Task 11. With points = ((1, 2), (3, 4)), print the second coordinate of the second point.

Solution

points = ((1, 2), (3, 4))
print(points[1][1])    # 4

Task 12. Try to change the first element of t = (1, 2, 3). What happens?

Answer

t[0] = 10 raises TypeError: 'tuple' object does not support item assignment.

Task 13. Convert [1, 2, 3] to a tuple, and (4, 5, 6) to a list.

Solution

print(tuple([1, 2, 3]))     # (1, 2, 3)
print(list((4, 5, 6)))      # [4, 5, 6]

Task 14. Create a one-item tuple containing 5 and check its type.

Solution

t = (5,)
print(type(t))     # <class 'tuple'>   (without the comma it would be int)

Task 15. Loop over ("ParottaSalna", "is", "good") and print each element.

Solution

for word in ("ParottaSalna", "is", "good"):
    print(word)

Task 16. Turn "hello" into a tuple of characters.

Solution

print(tuple("hello"))      # ('h', 'e', 'l', 'l', 'o')

Task 17. Turn d = {"one": 1, "two": 2} into a tuple of its items.

Solution

d = {"one": 1, "two": 2}
print(tuple(d.items()))    # (('one', 1), ('two', 2))

Task 18. Write a function that returns the sum of a tuple of numbers.

Solution

def tuple_sum(numbers):
    total = 0
    for n in numbers:
        total += n
    return total

print(tuple_sum((10, 20, 30)))    # 60   (or simply sum((10, 20, 30)))

Task 19. Use tuples as dictionary keys for points on a grid.

Solution

grid = {(0, 0): "origin", (1, 2): "point A", (3, 1): "point B"}
print(grid[(1, 2)])                 # point A
for (x, y), name in grid.items():
    print(f"{name} is at x={x}, y={y}")

This works because tuples are immutable (hashable); {[0, 0]: "origin"} would raise TypeError.