A setlist is an ordered, indexed collection with unique elements. It it more than just an ordered Set in that the elements are accessible by index (ie. not just a linked set).

However, setlist’s are not comparable like sets or lists. Equality testing still works, but setlist(('a', 'c')) < setlist(('a', 'b')) does not because we’d have to choose to compare by order or by set comparison.

There are two classes provided:

This is a mutable setlist
This is a frozen (implements version of a setlist.

Both classes implement,


>>> from collections_extended import setlist
>>> import string
>>> sl = setlist(string.ascii_lowercase)
>>> sl
setlist(('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'))
>>> sl[3]
>>> sl[-1]
>>> 'r' in sl   # testing for inclusion is fast
>>> sl.index('m')       # so is finding the index of an element
>>> sl.insert(1, 'd')   # inserting an element already in raises a ValueError
Traceback (most recent call last):
        raise ValueError
>>> sl.index('d')

Compared to existing similar implementations

Most implementations I’ve see are ordered sets where items are not accessible by index.

Compared to Standard Types

setlist vs. list

  • Inclusion testing is O(1)
  • Finding an element is O(1)
  • Adding an element that is already present raises a ValueError

setlist vs. set

  • Elements are ordered and accessible by index
  • Adding an element is as slow as adding to a list
    • Amortized O(n) for arbitrary insertions
    • O(1) for appending

New Methods

Aside from the methods expected from Sequence and Set, this provides: - setlist.shuffle(random=None) Because random.shuffle(setlist) doesn’t work, this is provided to do the same.


Some methods will raise a ValueError when trying to add or remove elements when they respectively already or do not currently exist in the setlist. Each method has an analagous version that does/doesn’t raise a ValueError. Methods implementing the Set methods do not raise ValueError while the one’s implementing do. All will raise TypeError if you use unhashable values. The bulk operations are atomic, if any single value is unhashable or a duplicate, no changes will be made to the setlist.

Raises ValueError No Yes
Interface Set Sequence
Add a single value add append
Add multiple values update extend
Remove a single value discard remove
Remove multiple values discard_all remove_all

The setlist constructor by defualt does not raise ValueError on duplicate values because we have to choose one or the other and this matches the behavior of Set. There is a flag raise_on_duplicate that can be passed to __init__ to raise a ValueError if duplicate values are aoassed.


  • Swapping elements, eg. sl[0], sl[1] = sl[1], sl[0], doesn’t work because
    it is implemented by first setting one element then the other. But since the first element it tries to set is still in the setlist, nothing happens. This causes random.shuffle not to work on a setlist.