all Python 3Sets (set)function

set.add

adding

Add an element to the set in place; duplicates are ignored.

Syntax

s.add(elem)

It returns None, so s = s.add(3) quietly turns your set into None — a classic slip. The element must be hashable: a list raises TypeError (unhashable type), so pass a tuple or a frozenset. Membership is decided by hash and ==, which makes 1, 1.0 and True the same element, and adding a duplicate leaves the stored object untouched.

Examples

s = {1, 2}
s.add(3)
print(sorted(s))   # → [1, 2, 3]
s.add(2)
print(sorted(s))   # → [1, 2, 3]

See also

Common errors

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