stableset 5.2.2

StableSet and OrderedSet are sets that remembers their order, and allows looking up its items by their index in that order.

A StableSet is a mutable set that remembers its insertion order. Featuring: Fast O(1) insertion, deletion, iteration and membership testing. But slow O(N) Index Lookup.

An OrderedSet is a mutable data structure that is a hybrid of a list and a set. It remembers its insertion order so that every entry has an index that can be looked up. Featuring: O(1) Index lookup, insertion, iteration and membership testing. But slow O(N) Deletion.

Both have similar interfaces but differ in respect of their implementation and performance.

Installation

ordered_set is available on PyPI and packaged as a wheel. You can list it as a dependency of your project, in whatever form that takes.

To install it into your current Python environment:

pip install ordered-set

To install the code for development, after checking out the repository:

pip install flit
flit install

Usage examples

An OrderedSet is created and used like a set:

>>> from ordered_set import OrderedSet

>>> letters = OrderedSet('abracadabra')

>>> letters
OrderedSet(['a', 'b', 'r', 'c', 'd'])

>>> 'r' in letters
True

It is efficient to find the index of an entry in an OrderedSet, or find an entry by its index. To help with this use case, the .add() method returns the index of the added item, whether it was already in the set or not.

>>> letters.index('r')
2

>>> letters[2]
'r'

>>> letters.add('r')
2

>>> letters.add('x')
5

OrderedSets implement the union (|), intersection (&), and difference (-) operators like sets do.

>>> letters |= OrderedSet('shazam')

>>> letters
OrderedSet(['a', 'b', 'r', 'c', 'd', 'x', 's', 'h', 'z', 'm'])

>>> letters & set('aeiou')
OrderedSet(['a'])

>>> letters -= 'abcd'

>>> letters
OrderedSet(['r', 'x', 's', 'h', 'z', 'm'])

The __getitem__() and index() methods have been extended to accept any iterable except a string, returning a list, to perform NumPy-like "fancy indexing".

>>> letters = OrderedSet('abracadabra')

>>> letters[[0, 2, 3]]
['a', 'r', 'c']

>>> letters.index(['a', 'r', 'c'])
[0, 2, 3]

OrderedSet implements __getstate__ and __setstate__ so it can be pickled, and implements the abstract base classes collections.MutableSet and collections.Sequence.

OrderedSet can be used as a generic collection type, similar to the collections in the typing module like List, Dict, and Set. For example, you can annotate a variable as having the type OrderedSet[str] or OrderedSet[Tuple[int, str]].

OrderedSet in data science applications

An OrderedSet can be used as a bi-directional mapping between a sparse vocabulary and dense index numbers. As of version 3.1, it accepts NumPy arrays of index numbers as well as lists.

This combination of features makes OrderedSet a simple implementation of many of the things that pandas.Index is used for, and many of its operations are faster than the equivalent pandas operations.

For further compatibility with pandas.Index, get_loc (the pandas method for looking up a single index) and get_indexer (the pandas method for fancy indexing in reverse) are both aliases for index (which handles both cases in OrderedSet).

Authors

OrderedSet was implemented by Elia Robyn Lake (maiden name: Robyn Speer). StableSet was implemented by Idan Miara, built upon the foundations of OrderedSet. Jon Crall contributed changes and tests to make it fit the Python set API. Roman Inflianskas added the original type annotations.

Comparisons

A StableSet is a mutable set that remembers its insertion order. Featuring: Fast O(1) insertion, deletion, iteration and membership testing. But slow O(N) Index Lookup.

An OrderedSet is a mutable data structure that is a hybrid of a list and a set. It remembers its insertion order so that every entry has an index that can be looked up. Featuring: O(1) Index lookup, insertion, iteration and membership testing. But slow O(N) Deletion.

Both have similar interfaces but differ in respect of their implementation and performance.

The original implementation of OrderedSet was a recipe posted to ActiveState Recipes by Raymond Hettiger, released under the MIT license.

Hettiger's implementation kept its content in a doubly-linked list referenced by a dict. As a result, looking up an item by its index was an O(N) operation, while deletion was O(1).

This version of OrderedSet makes different trade-offs for the sake of efficient lookups. Its content is a standard Python list instead of a doubly-linked list. This provides O(1) lookups by index at the expense of O(N) deletion, as well as slightly faster iteration.

Other implementations

The included implementation of OrderedSet is fully compatible with the following implementation:

• https://pypi.org/project/orderedset/ - by Simon Percivall (faster implementation of OrderedSet using Cython, which currently only works for Python<3.9)

The included implementation of StableSet is fully compatible with the following implementation:

• https://pypi.org/project/Ordered-set-37/ - by Xavier Bustamante Talavera (Similar basic implementation for StableSet, but named OrderedSet)