NotesPythonData Structures

Sets

Unordered collections of unique items — and the math-style operations that come with them.

A set stores unique, unordered items — duplicates are automatically dropped, and there's no indexing (my_set[0] doesn't work, since order isn't guaranteed).

tags = {"python", "tutorial", "python", "beginner"}
print(tags)          # {'python', 'tutorial', 'beginner'} -- duplicate dropped
print(len(tags))     # 3
tags.add("2024")
tags.discard("tutorial")   # remove if present, no error if missing

An empty {} creates a dict, not a set — for an empty set you must write set().

Why use a set?

  1. Deduplication — turn a list with duplicates into unique values fast: unique = list(set(my_list)).
  2. Fast membership checks — x in my_set is on average O(1), much faster than x in my_list for large collections.
  3. Set math — union, intersection, difference, exactly like set theory in math class.
watched = {"Intro", "Variables", "Loops"}
recommended = {"Loops", "Functions", "Decorators"}

print(watched | recommended)   # union: all unique videos from both
print(watched & recommended)   # intersection: videos in both sets -> {'Loops'}
print(watched - recommended)   # difference: watched but not recommended
print(watched ^ recommended)   # symmetric difference: in exactly one, not both

Deduplicating a list while keeping it simple

views_log = [120, 340, 120, 500, 340, 120]
unique_views = list(set(views_log))
print(sorted(unique_views))   # [120, 340, 500] -- set() doesn't preserve order, so sort if order matters
Key points to remember
  • •Sets store unique, unordered items — duplicates are dropped automatically, and there's no indexing.
  • •{} creates an empty dict, not an empty set — use set() for that.
  • •x in a_set is much faster on average than x in a_list for large collections.
  • •| is union, & is intersection, - is difference, ^ is symmetric difference — same as set theory in math.
  • •list(set(my_list)) is the quick idiom to deduplicate a list, but it doesn't preserve the original order.

Set math on two video playlists

Revision Flashcards

1 / 3

Tap a card to flip it and see the answer.

DictionariesComprehensions