Programming Fundamentals › Collections

Set

An unordered collection of unique values.

Also known as: hash set, unordered collection

A set is a collection of unique values with no particular order. Adding something that’s already there changes nothing.

tags = {"python", "sql", "python"}      # {"python", "sql"}
tags.add("git")
"sql" in tags                           # True, and fast
tags.discard("sql")

a = {1, 2, 3}
b = {3, 4}
a | b      # union         {1, 2, 3, 4}
a & b      # intersection  {3}
a - b      # difference    {1, 2}

const tags = new Set(["python", "sql", "python"]);
tags.has("sql");

Why use one

• Fast membership checks. x in my_set takes about the same time however big the set is, thanks to hashing. x in my_list scans the whole list.
• Removing duplicates: list(set(items)).
• Comparing groups: who’s in A but not B? Which users appear in both lists?

wanted = set(ids)                         # build once
[u for u in users if u.id in wanted]      # fast lookups

Things to remember

• No order, no indexing. You can’t ask for the third element. (Python sets don’t promise order. JavaScript’s Set remembers insertion order.)
• Items must be hashable. In Python, lists and dicts can’t go in a set, but tuples and strings can.
• {} is an empty dictionary in Python, not a set. Use set().
• Converting a list to a set to remove duplicates loses the original order. To keep order, use dict.fromkeys(items).