10-tuples-and-sets

10 · Tuples & Sets

Part 2 — Data Structures · Estimated time: 25–35 min · Prerequisite: 09 · Lists

Lists aren't the only way to group values. A tuple is like a list that can't be changed — perfect for fixed groups of data. A set is an unordered collection that automatically removes duplicates — perfect for "unique things" and for comparing groups.

What you'll learn

• Creating tuples and why they're immutable
• Unpacking a tuple into separate variables
• Creating sets, and how they drop duplicates
• Set membership and the fact that sets have no order / no index
• Set maths: union |, intersection &, difference -

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1. Tuples

A tuple uses parentheses and, once created, cannot be changed.

point = (3, 4)
print(point[0])      # 3       (index like a list)
print(len(point))    # 2
# point[0] = 9       # TypeError — tuples are immutable

# Unpacking: hand each item to its own variable.
x, y = point
print(x, y)          # 3 4

Use a tuple for things that belong together and shouldn't change — coordinates, a date as (year, month, day), an RGB colour.

Note: a one-item tuple needs a trailing comma — (5,), not (5).

▶️ Run it: python examples/01_tuples.py

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2. Sets

A set uses curly braces and keeps only unique values.

nums = {1, 2, 3, 2, 1}
print(nums)          # {1, 2, 3}  — duplicates gone (order may vary)
print(len(nums))     # 3

nums.add(4)          # add a value
nums.add(2)          # already present — no effect
nums.discard(1)      # remove a value (no error if it's missing)
print(2 in nums)     # True

Sets have no order, so there's no nums[0] — you can't index a set.

▶️ Run it: python examples/02_sets.py

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3. Set maths

Sets shine when comparing groups.

a = {1, 2, 3, 4}
b = {3, 4, 5}
print(sorted(a | b))   # [1, 2, 3, 4, 5]  union: in either
print(sorted(a & b))   # [3, 4]           intersection: in both
print(sorted(a - b))   # [1, 2]           difference: in a but not b

(We wrap results in sorted() only to print them in a predictable order.)

▶️ Run it: python examples/03_set_operations.py

💡 Try it yourself: given a list with repeats, how would you count how many distinct values it has? (Hint: turn it into a set.)

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Common mistakes

Mistake	What happens	Fix
point[0] = 9 on a tuple	TypeError	Tuples can't change; build a new tuple instead.
(5) expecting a tuple	that's just the number 5 in parentheses	A single-item tuple needs a comma: (5,).
my_set[0]	TypeError — sets aren't indexable	Sets are unordered; loop over them or convert to a list.
Relying on set print order	order isn't guaranteed	Use sorted(my_set) when order matters.

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Recap / cheat-sheet

# Tuple — ordered, immutable
t = (3, 4)
t[0]                 # 3
x, y = t             # unpack

# Set — unordered, unique
s = {1, 2, 2, 3}     # -> {1, 2, 3}
s.add(4)             # add
s.discard(1)         # remove (safe if missing)
v in s               # membership
a | b   a & b   a - b   # union / intersection / difference

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Exercises

Run a file with python exercises/<file>.py, then compare with the matching file in solutions/.

1. exercises/01_unpack_point.py — unpack a coordinate tuple.
2. exercises/02_unique_words.py — count the distinct words with a set.
3. exercises/03_common_friends.py — find friends two people share.

Try each yourself before opening the solution.

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