Sets

Sets store unique values. Use them when you care about membership or deduplication more than order.

A set is like a bag of distinct items. Adding the same value twice does not create a duplicate. If you only need to know whether something is present, a set usually fits better than a list.

Creating sets

tags = {"docs", "guide", "acton"}
empty_tags: set[str] = set()

Use {...} for a non-empty set. Use set() for an empty one, because {} means an empty dictionary.

Checking and updating

tags = {"docs", "guide"}

print("docs" in tags)
print("api" not in tags)

tags.add("api")
tags.add("docs")
tags.discard("guide")
tags.update({"reference", "tutorial"})

print(tags)
print(len(tags))

add() inserts one value. discard() removes a value if it is present and does nothing if it is not. update() adds values from another set or any iterable of values.

Removing values

tags = {"docs", "guide", "acton"}

print(tags.pop())
print(tags)

pop() removes and returns one element. On an empty set it raises an exception, so check before calling it if the set may be empty.

Iteration and order

tags = {"docs", "guide", "acton"}

for tag in tags:
    print(tag)

print(sorted(tags))

Sets do not preserve order. Do not rely on the printed order when the exact sequence matters. If you need stable display order, sort the set first.

Set comprehensions

words = ["hello", "world", "hello", "acton"]
unique_words = {word for word in words}
long_words = {word.upper() for word in words if len(word) > 4}
remainder_classes = {n % 3 for n in range(10)}

Set comprehensions are a compact way to build a set while automatically removing duplicates.

Freezing a set

Use freeze(tags) to convert a set to an immutable iset in place:

tags = {"docs", "guide"}
tags.add("api")
frozen_tags = freeze(tags)

The conversion takes constant time. It changes the object's class and keeps the same hash table and elements. The resulting iset supports membership, iteration, set operations, and hashing, so it can be a dictionary key or an element of another set. Sets implement Freeze[iset[A]], which lets the global freeze() function return the corresponding immutable set type.

After calling freeze(), use only the returned iset. The original set and any aliases must no longer be used. The compiler does not yet enforce this ownership requirement; escape and reachability analysis will check it in a future version.

Use iset(tags) to make an immutable copy while keeping a mutable set, or set(frozen_tags) to make a new mutable copy.

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