RuleTwoRound

class whalrus.RuleTwoRound(*args, rule1: whalrus.rules.rule.Rule = None, rule2: whalrus.rules.rule.Rule = None, elimination: whalrus.eliminations.elimination.Elimination = None, **kwargs)

The two-round system.

Parameters

• args – Cf. parent class.

• rule1 – The first rule. Default: RulePlurality.

• rule2 – The second rule. Default: RulePlurality.

• elimination (Elimination) – The elimination algorithm used during the first round. Default: EliminationLast with k=-2, which only keeps the 2 best candidates.

• kwargs – Cf. parent class.

Examples

With its default settings, this class implements the classic two-round system, using plurality at both rounds:

>>> rule = RuleTwoRound(['a > b > c > d > e', 'b > a > c > d > e', 'c > a > b > d > e'],
...                     weights=[2, 2, 1])
>>> rule.first_round_.rule_.gross_scores_
{'a': 2, 'b': 2, 'c': 1, 'd': 0, 'e': 0}
>>> rule.second_round_.gross_scores_
{'a': 3, 'b': 2}

Using the options, some more exotic two-round systems can be defined, such as changing the rule of a round:

>>> rule = RuleTwoRound(['a > b > c > d > e', 'b > a > c > d > e', 'c > a > b > d > e'],
...                     weights=[2, 2, 1], rule1=RuleBorda())
>>> rule.first_round_.rule_.gross_scores_
{'a': 17, 'b': 16, 'c': 12, 'd': 5, 'e': 0}
>>> rule.second_round_.gross_scores_
{'a': 3, 'b': 2}

… or changing the elimination algorithm:

>>> rule = RuleTwoRound(['a > b > c > d > e', 'b > a > c > d > e', 'c > a > b > d > e'],
...                     weights=[2, 2, 1], elimination=EliminationLast(k=-3))
>>> rule.first_round_.rule_.gross_scores_
{'a': 2, 'b': 2, 'c': 1, 'd': 0, 'e': 0}
>>> rule.second_round_.gross_scores_
{'a': 2, 'b': 2, 'c': 1}

property cotrailers_

“Cotrailers” of the election, i.e. the candidates that fare worst in the election. This is the last equivalence class in order_. For example, in RuleScoreNum, it is the candidates that are tied for the worst score.

Type

NiceSet

property cowinners_

Cowinners of the election, i.e. the candidates that fare best in the election.. This is the first equivalence class in order_. For example, in RuleScoreNum, it is the candidates that are tied for the best score.

Type

NiceSet

property elimination_rounds_

The elimination rounds. A list of Elimination objects. All rounds except the last one.

Type

list

property final_round_

The final round, which decides the winner of the election.

Type

Rule

property first_round_

The first round. This is just a shortcut for self.elimination_rounds_[0].

Type

Elimination

property n_candidates_

Number of candidates.

Type

int

property rounds_

The rounds. All rounds but the last one are Elimination objects. The last one is a Rule object.

Examples

Note that in some cases, there may be fewer actual rounds than declared in the definition of the rule:

>>> rule = RuleSequentialElimination(
...     ['a > b > c > d', 'a > c > d > b', 'a > d > b > c'],
...     rules=[RuleBorda(), RulePlurality(), RulePlurality()],
...     eliminations=[EliminationBelowAverage(), EliminationLast(k=1)])
>>> len(rule.rounds_)
2
>>> rule.elimination_rounds_[0].rule_.gross_scores_
{'a': 9, 'b': 3, 'c': 3, 'd': 3}
>>> rule.final_round_.gross_scores_
{'a': 3}

Type

list

property second_round_

The second round. This is just an alternative name for self.final_round_.

Type

Rule

property strict_order_

Result of the election as a strict order over the candidates. The first element is the winner, etc. This may use the tie-breaking rule.

Type

list

property trailer_

The “trailer” of the election. This is the last candidate in strict_order_ and also the unfavorable choice of the tie-breaking rule in cotrailers_.

Type

object

property winner_

The winner of the election. This is the first candidate in strict_order_ and also the choice of the tie-breaking rule in cowinners_.

Type

object