orderedsets 2026.1

An implementation of mutable and immutable ordered sets.

orderedsets

An implementation of mutable and immutable ordered sets as thin wrappers around Python's dict class. These classes are meant as drop-in replacements for Python's builtin set and frozenset classes. Care has been taken to provide the same functionality as the Python classes, without API additions or removals, to allow easy switching between set implementations. These classes are often faster than other ordered set implementations (but slower than Python's builtin sets).

In contrast to Python's builtin set and frozenset classes, the order of items is kept (generally, insertion order), such that iterating over items in the set as well as mutating operations are deterministic.

This package has no external dependencies.

Usage

Install this package with:

$ pip install orderedsets

Usage example:

from orderedsets import OrderedSet, FrozenOrderedSet

os = OrderedSet([1, 2, 4])
os.add(0)
assert list(os) == [1, 2, 4, 0]
os.remove(0)

fos = FrozenOrderedSet([1, 2, 4])
# a.add(0)  # raises AttributeError: 'FrozenOrderedSet' object has no attribute 'add'
assert list(fos) == [1, 2, 4]

# sets with the same elements compare equal
assert os == fos == set([1, 2, 4]) == frozenset([1, 2, 4])

# only immutable sets can be hashed
assert hash(fos) == hash(frozenset([1, 2, 4]))

Please also see the documentation.

References

Other packages

• https://github.com/rindPHI/proxyorderedset/ (not 100% compatible with set)
• https://pypi.org/project/ordered-set/ (no frozen/immutable class)
• https://pypi.org/project/stableset/ (no frozen/immutable class)
• https://pypi.org/project/orderedset/ (Cython, no frozen/immutable class)
• https://pypi.org/project/Ordered-set-37/ (no frozen/immutable class)
• https://pypi.org/project/sortedcollections (no frozen/immutable class)
• https://github.com/grantjenks/python-sortedcollections (no frozen/immutable class)
• https://github.com/Erotemic/ubelt (pure Python, no frozen/immutable class)

Discussions

• https://discuss.python.org/t/add-orderedset-to-stdlib/12730

Python implementations

• https://github.com/python/cpython/blob/main/Objects/setobject.c
• https://github.com/python/cpython/blob/main/Objects/dictobject.c