Sets in Python

Creating Sets

s = {1, 2, 3}
s = set([1, 2, 2, 3])  # {1, 2, 3} - removes duplicates
empty = set()          # NOT {} (that's empty dict!)

Properties

• Unordered - no indexing
• Unique elements only
• Elements must be hashable

Adding/Removing

• add(x) - Add single element
• update(iterable) - Add multiple
• remove(x) - Remove (KeyError if missing)
• discard(x) - Remove (no error if missing)
• pop() - Remove and return arbitrary element

Set Operations

a | b    # Union
a & b    # Intersection
a - b    # Difference
a ^ b    # Symmetric difference

Comparisons

• a <= b or a.issubset(b) - Subset
• a >= b or a.issuperset(b) - Superset
• a.isdisjoint(b) - No common elements

Frozenset

Immutable set, can be dict key:

fs = frozenset([1, 2, 3])

Key Points

• {} creates dict, not set! Use set()
• Elements must be hashable
• remove() raises error, discard() doesn’t
• Operators: | union, & intersection, - difference