A set is a mutable and unordered collection of hashable objects.
Set members must be distinct — duplicate items are not allowed.
They can hold multiple different data types and even nested structures like a tuple of tuples — as long as all elements can be hashed.
Sets also come in an immutable frozensets flavor.
Sets are most commonly used to quickly remove duplicates from other data structures or item groupings. They are also used for efficient comparisons when sequencing and duplicate tracking are not needed.
Like other collection types (dictionaries, lists, tuples), sets support:
- Iteration via
for item in <set> - Membership checking via
inandnot in, - Length calculation through
len(), and - Shallow copies through
copy()
sets do not support:
- Indexing of any kind
- Ordering via sorting or insertion
- Slicing
- Concatenation via
+
Checking membership in a set has constant time complexity (on average) versus checking membership in a list or string, where the time complexity grows as the length of the data increases.
Methods such as <set>.union(), <set>.intersection(), or <set>.difference() also have constant time complexity (on average).