CODESARRAY

Lesson 6, by Codesarray, , 5 min read

Python tuples and sets: unpacking, swapping and removing duplicates

Make a Python tuple, unpack it, swap two variables in one line, and use a set to remove duplicates and check membership in O(1).

A tuple is a list that never changes, and a set is a collection that holds each value once. By the end of this page you will make a tuple, unpack it into names, swap two variables in one line, remove duplicates with a set, and know why the in test is so much faster on a set than on a list.

Tuples: round brackets, never change

A tuple is written with round brackets, and you index it just like a list: point = (3, 4) gives point[0] as 3. The difference is that it can never change. point[0] = 9 raises TypeError: 'tuple' object does not support item assignment. Use a tuple for a small, fixed record, such as a coordinate, where the pieces belong together.

Unpacking and swapping

Unpacking pulls a tuple apart into names: x, y = point gives x the 3 and y the 4. The number of names has to match the number of items. The same idea swaps two variables in one line: a, b = b, a. No temporary variable is needed.

Sets: each value once, no order

A set holds each value once. Pass a list to set() and the repeats are gone. A set has no order, so you cannot index it: {1, 2}[0] raises TypeError: 'set' object is not subscriptable. add() puts a new item in, and adding one that is already there changes nothing.

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

tags = ["py", "go", "py", "js", "go"]
unique = set(tags)
print(len(unique))
print(sorted(unique))
print("js" in unique)
3 4
4 3
3
['go', 'js', 'py']
True

Line 2 unpacks the tuple and line 3 prints 3 4. Line 4 swaps the two and line 5 prints 4 3. Line 8 turns the list with five items into a set, and line 9 shows only three are left. Line 10 prints them sorted, because a set has no order of its own to print. Line 11 asks whether "js" is in the set.

Why in is fast on a set

Asking whether "js" is in a list checks the items one by one, so it is O(n). A set jumps straight to the answer, O(1), even with a million items, because it uses a hash table: the value itself points to its slot. That is why a set is the fast way to check membership, and why turning a list into a set is the quick way to remove duplicates.

What you learned

  • A tuple is an ordered collection in round brackets that never changes.
  • x, y = point unpacks a tuple, and a, b = b, a swaps two values.
  • A set holds each value once, with no order, so you cannot index it.
  • set(tags) removes duplicates, and add() puts a new item in.
  • in is O(1) on a set and O(n) on a list.

Common questions

How do I make an empty set? Use set(). Empty curly braces, {}, make an empty dictionary, not a set.

How do I make a tuple with one item? Add a trailing comma: (5,) is a tuple, while (5) is just the number 5.

**Does set() keep the order of my list?** No. If you need the duplicates gone and the first-seen order kept, use list(dict.fromkeys(tags)), which gives ['py', 'go', 'js'].

Can a tuple contain a list? Yes, and the list inside can still change. The tuple only guarantees that it keeps pointing at the same items.

Can I use a tuple as a set item? Yes. Tuples never change, so they can go in a set. Lists cannot.

This lesson follows on from Python lists. Next in the series: dictionaries.

This article goes with the video Tuples and sets. Watch it on YouTube.