12.8 Set Methods

Method equivalents of the operators in 12.7 Set Operators, plus relationship-testing methods that have no operator form.

union()

Same as | — method form is useful when chaining or passing as a function argument.

>>> {1, 2, 3}.union({2, 3, 4})
{1, 2, 3, 4}

intersection()

>>> {1, 2, 3}.intersection({2, 3, 4})
{2, 3}

difference()

>>> {1, 2, 3}.difference({2, 3, 4})
{1}

symmetric_difference()

>>> {1, 2, 3}.symmetric_difference({2, 3, 4})
{1, 4}

issubset()

True if every element of this set is also in the other set.

>>> {1, 2}.issubset({1, 2, 3})
True

issuperset()

True if this set contains every element of the other set — the inverse relationship of issubset().

>>> {1, 2, 3}.issuperset({1, 2})
True

isdisjoint()

True if the two sets share no elements at all.

>>> {1, 2}.isdisjoint({3, 4})
True

copy()

Returns a shallow copy — a new set object with the same elements.

>>> a = {1, 2, 3}
>>> c = a.copy()
>>> c is a, c == a
(False, True)

Quick Interview Answer

“Every operator from the previous section has a method twin — union(), intersection(), difference(), symmetric_difference() — useful mainly when chaining or passing as a function argument, where an operator can’t be used directly. The methods with no operator equivalent are the relationship tests: issubset() and issuperset() are inverses of each other, and isdisjoint() checks for zero overlap. copy() returns a genuine shallow copy — unlike a tuple’s copy.copy(), which can return the identical object since tuples are immutable, a set’s copy() always produces a distinct object, since a set is mutable and the copy must be safe to modify independently.”

Common Mistakes

• Confusing issubset() and issuperset() — a.issubset(b) asks “is everything in a also in b?”; a.issuperset(b) asks the reverse.
• Using a.difference(b) and expecting it to also remove those elements from a — it returns a new set; use difference_update() if in-place removal is actually wanted.
• Assuming copy() performs a deep copy — like any shallow copy, if the set held mutable objects (which it normally can’t, since elements must be hashable) only the top-level container would be duplicated.