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Extra Python Data Structures - bags/multisets and setlists

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Getting Started

>>> from collections_extended import bag, setlist
>>> b = bag('abracadabra')
>>> b.count('a')
>>> sl = setlist('abracadabra')
>>> sl[3]
>>> sl[-1]


pip install data-structures


This package includes one module - collections_extended. This module extends the built-in collections module to include a bag class, AKA multiset, and a setlist class, which is a list of unique elements or an ordered set depending on how you look at it. There are also frozen (hashable) varieties of each included. Finally, all collections are abstracted into one Collection abstract base class and a Collection factory is provided where you can create a Collection by specifying the properties unique, ordered and mutable.


from collections_extended import bag, frozenbag, setlist, frozensetlist


There are four new classes provided:

This is a bag AKA multiset.
This is a frozen (hashable) version of a bag.
An ordered set or a list of unique elements depending on how you look at it.
This is a frozen (hashable) version of a setlist.


Bags have constant time inclusion testing but can only contain hashable elements. See

  • count(elem)
    Returns the count of elem in the bag. O(1)
  • num_unique_elements()
    Returns the number of unique elements in the bag. O(1)
  • unique_elements()
    Returns a set of all the unique elements in the bag. O(1)
  • nlargest(n=None)
    Returns the n most common elements and their counts from most common to least. If n is None then all elements are returned. O(n log n)
  • copy()
    Returns a shallow copy of self. O(self.num_unique_elements())
  • cardinality()
    Returns the cardinality of this bag. Same as len(self). O(1)
  • underlying_set()
    Returns the underlying set. Same as self.unique_elements().
  • multiplicity(elem)
    Same as self.count(elem)
  • isdisjoint(other: Iterable)
    Tests if self is disjoint with any other Iterable. O(len(other))

The following are only for mutable bags (not frozenbags).

  • pop()
  • add(elem)
  • discard(elem)
  • remove(elem)
  • clear()


A setlist is an ordered collection with unique elements. The class implements Sequence and Set and should be able to be used as a drop in replacement for a set or list of you want to add the add an additional constraint of ordering or uniqueness. It it more than just an ordered Set in that the elements are accessible by index (ie. not just a linked set).

Collection Factory

A Collection factory is provided where you can specify whether you want the Collection returned to be mutable, have unique elements and/or be ordered. If an Iterable object is passed the Collection will be filled from it, otherwise it will be empty.

collection(it = None, mutable=True, unique=False, ordered=False)

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