Python 3.3+collectionsterm

collections.abc.Set

collections.abc — protocols

The abstract base class for read-only sets: implement __contains__, __iter__ and __len__ and you get &, |, -, ^, comparisons and isdisjoint.

Syntax

class MySet(collections.abc.Set):
    def __contains__(self, x): ...
    def __iter__(self): ...
    def __len__(self): ...

The three methods buy you operators, not named methods: union(), intersection() and add() do not appear — they are the set API, not Set. The results of &, | and - are built by the classmethod _from_iterable, which calls cls(iterable), so override it if your constructor takes other arguments. Because Set defines __eq__, your class becomes unhashable; set __hash__ = Set._hash to use it as a dict key. set, frozenset, dict.keys() and dict.items() are all Sets, but dict.values() and list are not.

Examples

from collections.abc import Set
class Ordered(Set):
    def __init__(self, items):
        self._items = list(dict.fromkeys(items))
    def __contains__(self, x):
        return x in self._items
    def __iter__(self):
        return iter(self._items)
    def __len__(self):
        return len(self._items)
a, b = Ordered([1, 2, 3]), Ordered([2, 3, 4])
print(list(a & b), list(a | b), list(a - b))  # → [2, 3] [1, 2, 3, 4] [1]
print(Ordered([1, 2]) <= a, a == {1, 2, 3})  # → True True
print(hasattr(a, 'union'), a.__hash__)  # → False None

from collections.abc import Set
d = {'a': 1}
print(isinstance(frozenset(), Set), isinstance(d.keys(), Set))  # → True True
print(isinstance(d.values(), Set), isinstance([], Set))  # → False False

collections.abc.Set

Syntax

Examples

See also