flexstv is a flexible implementation of the Single Transferrable Vote (STV) algorithm in Python by Lorenzo Losa. Unlike most STV implementations, it allows tied rankings, prorating the vote for each candidate in the tie by the number of candidates so-ranked which remain in each round.
License: GPLv3
Requires Python 3.
Votes are specified as a set of Cast Vote Records (CVRs) formatted as a comma-separate values (csv) file. The first header row names the candidates. The first column of the file provides a unique id for the voter. The last occurrence of a vote in the file for a given voter overrides previous votes.
In the the remaining columns appear the ranks (where 1 is the most-preferred) given by the voter to the candidate. Equal ranks are allowed, and result in prorated votes.
See votes.csv for a contest corresponding to the example at Wikipedia: Single transferable vote.
flexstv [-h] [-s SEATS] [-r] [-v VERBOSITY] cvr_file
The number of seats (winners) to assign defaults to 2 and the verbosity defaults to 2 (highest).
Reproduce the results from Wikipedia:
$ flexstv.py -s 3 -v 1 votes.csv
Counting the votes...
Total votes: 20
Quota: 6
Step 1. Chocolate (12.00); Orange (4.00); Pear (2.00); Strawberry (1.00); Sweets (1.00)
Chocolate is selected.
Step 2. Strawberry (5.00); Orange (4.00); Sweets (3.00); Pear (2.00)
Pear loses.
Step 3. Orange (6.00); Strawberry (5.00); Sweets (3.00)
Orange is selected.
Step 4. Strawberry (5.00); Sweets (3.00)
Sweets loses.
Step 5. Strawberry (8.00)
Strawberry is selected.
Example Contest with some equal rankings:
$ flexstv.py -s 2 -v 1 some-equal-rankings.csv
Counting the votes...
Total votes: 18
Quota: 7
Step 1. A (7.67); B (4.17); C (3.83); E (2.00); D (0.33)
A is selected.
Step 2. B (4.41); C (3.98); E (2.13); D (0.48)
D loses.
Step 3. B (4.80); C (4.02); E (2.17)
E loses.
Step 4. B (5.89); C (5.11)
C loses.
Step 5. B (11.00)
B is selected.