Tuples in Python

Learning objectives

By the end of this pill you will be able to:

• create and initialise tuples (tuple);
• access elements by index;
• understand tuple immutability;
• use tuples for multiple assignments and as dictionary keys;
• clearly distinguish tuples from lists and sets.

What is a tuple

A tuple is an ordered and immutable collection of elements. It is created with round parentheses ( ) or with the tuple() function.

coordinates = (10, 20)
colours = ("red", "green", "blue")
empty = ()
singleton = (5,)     # tuple with a single element

Note: Tuples can contain heterogeneous elements and even other tuples.

Accessing elements

Just like lists, indices are used (starting from 0).

t = ("a", "b", "c", "d")
print(t[0])   # a
print(t[2])   # c
print(t[-1])  # d (last element)

Immutability

Once created, a tuple cannot be modified: you cannot add, delete or change elements. This makes it “safer” when you don’t want data to be altered.

t = (1, 2, 3)
# t[0] = 99    # ERROR: tuple objects do not support item assignment

Note: You can however create a new tuple from existing ones:

t = (1, 2, 3)
new = t + (4, 5)
print(new)  # (1, 2, 3, 4, 5)

Multiple assignments

Tuples are widely used for unpacking and multiple assignments.

point = (3, 4)
x, y = point
print(x, y)    # 3 4

a, b, c = (10, 20, 30)
print(a+b+c)   # 60

Tuples as dictionary keys

Because they are immutable, tuples can be used as keys in a dictionary (which is not possible with lists).

contacts = {
    ("Mario", "Rossi"): "333-1234567",
    ("Luca", "Bianchi"): "333-7654321"
}
print(contacts[("Mario", "Rossi")])

Useful functions

• len(t) → length of the tuple.
• min(t), max(t) → minimum and maximum (if comparable).
• tuple(list) → converts a list to a tuple.
• slicing → works like with lists: t[1:3].

t = (10, 20, 30, 40)
print(len(t))            # 4
print(t[1:3])            # (20, 30)
print(tuple([1, 2, 3]))  # (1, 2, 3)

Differences from lists and sets

• Lists: ordered, mutable, allow duplicates.
• Sets: unordered, mutable, eliminate duplicates, not indexable.
• Tuples: ordered, immutable, allow duplicates, indexable.

lst   = [1, 2, 2, 3]
s     = {1, 2, 2, 3}
tup   = (1, 2, 2, 3)
print(lst)    # [1, 2, 2, 3]
print(s)      # {1, 2, 3}
print(tup)    # (1, 2, 2, 3)

In brief: use a tuple when you need an ordered, immutable collection of values — useful for representing coordinates, dates or fixed records.