Tutorial¶

>>> from whalrus import *

Quick start¶

Some simple elections:

>>> RulePlurality(['a', 'a', 'b', 'c']).winner_
'a'
>>> RuleBorda(['a > b > c', 'b > c > a']).gross_scores_
{'a': 2, 'b': 3, 'c': 1}

Elections can optionally have weights and voter names:

>>> RulePlurality(
...     ['a', 'a', 'b', 'c'], weights=[1, 1, 3, 2],
...     voters=['Alice', 'Bob', 'Cate', 'Dave']
... ).winner_
'b'

The tie-breaking rule can be specified:

>>> RulePlurality(['a', 'a', 'b', 'b', 'c'], tie_break=Priority.ASCENDING).winner_
'a'

Computed attributes of an election¶

>>> plurality = RulePlurality(['a', 'a', 'b', 'b', 'c'], tie_break=Priority.ASCENDING)

Once the election is defined, you can access its computed attributes, whose names end with an underscore:

>>> plurality.candidates_
{'a', 'b', 'c'}
>>> plurality.gross_scores_
{'a': 2, 'b': 2, 'c': 1}
>>> plurality.scores_
{'a': Fraction(2, 5), 'b': Fraction(2, 5), 'c': Fraction(1, 5)}
>>> plurality.best_score_
Fraction(2, 5)
>>> plurality.worst_score_
Fraction(1, 5)
>>> plurality.order_
[{'a', 'b'}, {'c'}]
>>> plurality.strict_order_
['a', 'b', 'c']
>>> plurality.cowinners_
{'a', 'b'}
>>> plurality.winner_
'a'
>>> plurality.cotrailers_
{'c'}
>>> plurality.trailer_
'c'

General syntax¶

In the most general syntax, firstly, you define the rule and enter its options:

>>> plurality = RulePlurality(tie_break=Priority.ASCENDING)

Secondly, you use it as a callable to load a particular election (profile, set of candidates):

>>> plurality(ballots=['a', 'b', 'c'], weights=[2, 2, 1], voters=['Alice', 'Bob', 'Cate'],
...           candidates={'a', 'b', 'c', 'd'})  # doctest:+ELLIPSIS
<... object at ...>

Finally, you can access the computed variables:

>>> plurality.gross_scores_
{'a': 2, 'b': 2, 'c': 1, 'd': 0}

Later, if you wish, you can load another profile with the same voting rule, and so on.

Under the hood¶

A whalrus.Ballot contains the message emitted by the voter, but also some contextual information such as the set of candidates that were available at the moment when she cast her ballot:

>>> ballot = BallotOrder('a > b ~ c')
>>> ballot
BallotOrder(['a', {'b', 'c'}], candidates={'a', 'b', 'c'})

This architecture allows Whalrus to deal with asynchronous elections where the set of candidates may vary during the election itself (such as some asynchronous online polls).

A whalrus.Profile contains a list of whalrus.Ballot objects, a list of weights and a list of voters:

>>> profile = Profile(['a > b ~ c', 'a ~ b > c'])
>>> profile.ballots
BallotOrder(['a', {'b', 'c'}], candidates={'a', 'b', 'c'})
>>> profile.weights
[1, 1]
>>> profile.voters
[None, None]

Internally, a voting rule is always applied to a whalrus.Profile. Hence, if the inputs are given in a “loose” format, they are converted to a whalrus.Profile:

>>> borda = RuleBorda(['a > b ~ c', 'a ~ b > c'])
>>> borda.profile_converted_  # doctest:+ELLIPSIS
Profile(ballots=[BallotOrder(['a', {'b', 'c'}], candidates={'a', 'b', 'c'}), ...)

Under the hood, some conversions are performed so that a variety of inputs are understood by Whalrus. In the example above, the first ballot was manually entered as a > b ~ c. In the absence of other information, Whalrus then considered that only candidates a, b and c were available when this voter cast her ballot. If you want to give more detailed information, the most general syntax consists in using the constructors of classes whalrus.Profile, whalrus.Ballot and their subclasses:

>>> a_more_complex_ballot = BallotOrder('a > b ~ c', candidates={'a', 'b', 'c', 'd', 'e'})

The ballot above means that the voter emitted the message ‘a > b ~ c’ in a context where the candidates d and e where also available, i.e. she deliberately abstained about these two candidates.

Change the candidates¶

It is possible to change the set of candidates, compared to when the voters cast their ballots.

>>> profile = Profile(['a > b > c', 'a ~ b > c'])
>>> RulePlurality(profile, candidates={'b', 'c'}).gross_scores_
{'b': 2, 'c': 0}