STVPoll 0.4.3

STV polling methods

STVPoll README

Library to perform STV Poll calculations. The package was created as part of the VoteIT project, specifically to handle larger elections that was hard to count with Markus Schulzes STV method.

Typical usage would be primary elections or elections with many winners from a pool of many candidates. The result will be proportional.

Fully supported:

• Scottish STV

• Instant-Runoff Voting (IRV)

Mostly working:

• CPO STV (Does not fail gracefully if too many outcomes)

Example

Case from: https://en.wikipedia.org/wiki/Single_transferable_vote

from stvpoll.scottish_stv import ScottishSTV

example_ballots = (
    (['orange'], 4),
    (['pear', 'orange'], 2),
    (['chocolate', 'strawberry'], 8),
    (['chocolate', 'bonbon'], 4),
    (['strawberry'], 1),
    (['bonbon'], 1),
)

poll = ScottishSTV(seats=3, candidates=['orange', 'chocolate', 'pear', 'strawberry', 'bonbon'])
for (candidates, count) in example_ballots:
    poll.add_ballot(candidates, count)

result = poll.calculate()

This will return a ElectionResult object that contains the result and some useful metadata. The elected attribute contains the elected candidates.

Candidates to the left have higher preference, so: [‘pear’, ‘orange’] means ‘pear’ before ‘orange’ etc. The number is how many of that kind of ballot there is.

result.elected_as_tuple()
('chocolate', 'orange', 'strawberry')

Code & Contributions

You may fork the code at: https://github.com/VoteIT/STVPoll

Please report any bugs there, or email [e-mail]

Changes

0.4.3 (2023-03-09)

• Fix subtle bug in vote transfers, where current votes was not updated for every vote transfer.

0.4.2 (2023-03-09)

• Fix bug in 0.4.1, where not all elected candidates votes were transferred to next preference.

0.4.1 (2023-03-09)

Important breaking changes: Result data structure is updated. Import paths changed.

• Rewritten type annotations.

• Rewritten internal data structures, minimizing mutation of data.

• Introducing tiebreak strategies.

• Random tiebreaks not uses a shuffled candidate list, which is reported as part of result.

• Utility to recalculate result from previous polls, using ordered candidate list in randomized tiebreaks.

• STVPollBase class moved from __init__.py to abcs.py.

• Updated result data structure.

0.3.0 (dev)

• Introduce pedantic_order=False, to avoid incomplete results when randomization can not affect who is elected.

0.2.3 (dev)

• Fixed bug where votes were discarded in select_multiple.

• Scottish STV: Select proposals in order of most votes, when there is no more competition.

0.2.2 (dev)

• Fixed bug with tie in first round on Scottish STV.

• Fixed bug in quota selection of all seats in CPO.

0.2.1 (dev)

• Unreleased

0.2.0 (2018-05-23)

• Fixed bug in deciding which vote to transfer first when multiple elected in Scottish STV. [schyffel] [robinharms]

• Scottish STV: Resolve ties so that winners are always in correct ranking order (extreme case). [schyffel]

• Now works on Python 3. [schyffel]

• Test coverage 100 %. [schyffel]

0.1.4 (2018-05-12)

• Fixed a situation where primary_candidate in rounds didn’t exist. [schyffel] [robinharms]

0.1.3 (2018-03-22)

• Excluded empty ballots, so that they do not affect the quota. [schyffel]

0.1.2 (2017-11-24)

• Fixed exception on empty ballots. [schyffel]

0.1.1 (2017-11-24)

• Fixed case where randomization caused an exception. [schyffel]

0.1.0 (2017-11-03)

• Initial version