Edit docs for metrics, partition, and proposals modules

Author: peterrrock2

gerrychain/metrics/compactness.py

@@ -1,15 +1,36 @@
 import math
+from typing import Dict
 
 
-def compute_polsby_popper(area, perimeter):
+def compute_polsby_popper(area: float, perimeter: float) -> float:
+    """
+    Computes the Polsby-Popper score for a single district.
+
+    :param area: The area of the district
+    :type area: float
+    :param perimeter: The perimeter of the district
+    :type perimeter: float
+
+    :returns: The Polsby-Popper score for the district
+    :rtype: float
+    """
     try:
         return 4 * math.pi * area / perimeter ** 2
     except ZeroDivisionError:
         return math.nan
 
 
-def polsby_popper(partition):
-    """Computes Polsby-Popper compactness scores for each district in the partition.
+# Partition type hint left out due to circular import
+# def polsby_popper(partition: Partition) -> Dict[int, float]:
+def polsby_popper(partition) -> Dict[int, float]:
+    """
+    Computes Polsby-Popper compactness scores for each district in the partition.
+
+    :param partition: The partition to compute scores for
+    :type partition: Partition
+
+    :returns: A dictionary mapping each district ID to its Polsby-Popper score
+    :rtype: Dict[int, float]
     """
     return {
         part: compute_polsby_popper(

gerrychain/metrics/partisan.py

@@ -1,26 +1,47 @@
+"""
+The partisan metrics in this file are later used in the module
+gerrychain.updaters.election.py. Thus, all of the election
+results objects here are implicilty typed as ElectionResults,
+but cannot be given an explicit type annotation due to problems
+with circular imports.
+"""
+
 import numpy
+from typing import Tuple
 
 
-def mean_median(election_results):
+def mean_median(election_results) -> float:
     """
     Computes the Mean-Median score for the given ElectionResults.
     A positive value indicates an advantage for the first party listed
     in the Election's parties_to_columns dictionary.
+
+    :param election_results: An ElectionResults object
+    :type election_results: ElectionResults
+
+    :returns: The Mean-Median score for the given ElectionResults
+    :rtype: float
     """
     first_party = election_results.election.parties[0]
     data = election_results.percents(first_party)
 
     return numpy.median(data) - numpy.mean(data)
 
 
-def mean_thirdian(election_results):
+def mean_thirdian(election_results) -> float:
     """
     Computes the Mean-Median score for the given ElectionResults.
     A positive value indicates an advantage for the first party listed
     in the Election's parties_to_columns dictionary.
 
     The motivation for this score is that the minority party in many
     states struggles to win even a third of the seats.
+
+    :param election_results: An ElectionResults object
+    :type election_results: ElectionResults
+
+    :returns: The Mean-Thirdian score for the given ElectionResults
+    :rtype: float
     """
     first_party = election_results.election.parties[0]
     data = election_results.percents(first_party)
@@ -31,24 +52,35 @@ def mean_thirdian(election_results):
     return thirdian - numpy.mean(data)
 
 
-def efficiency_gap(results):
+def efficiency_gap(election_results) -> float:
     """
     Computes the efficiency gap for the given ElectionResults.
     A positive value indicates an advantage for the first party listed
     in the Election's parties_to_columns dictionary.
+
+    :param election_results: An ElectionResults object
+    :type election_results: ElectionResults
+
+    :returns: The efficiency gap for the given ElectionResults
+    :rtype: float
     """
-    party1, party2 = [results.counts(party) for party in results.election.parties]
+    party1, party2 = [election_results.counts(party) for party in election_results.election.parties]
     wasted_votes_by_part = map(wasted_votes, party1, party2)
-    total_votes = results.total_votes()
+    total_votes = election_results.total_votes()
     numerator = sum(waste2 - waste1 for waste1, waste2 in wasted_votes_by_part)
     return numerator / total_votes
 
 
-def wasted_votes(party1_votes, party2_votes):
+def wasted_votes(party1_votes: int, party2_votes: int) -> Tuple[int, int]:
     """
     Computes the wasted votes for each party in the given race.
-    :party1_votes: the number of votes party1 received in the race
-    :party2_votes: the number of votes party2 received in the race
+    :param party1_votes: the number of votes party1 received in the race
+    :type party1_votes: int
+    :param party2_votes: the number of votes party2 received in the race
+    :type party2_votes: int
+
+    :returns: a tuple of the wasted votes for each party
+    :rtype: Tuple[int, int]
     """
     total_votes = party1_votes + party2_votes
     if party1_votes > party2_votes:
@@ -60,12 +92,18 @@ def wasted_votes(party1_votes, party2_votes):
     return party1_waste, party2_waste
 
 
-def partisan_bias(election_results):
+def partisan_bias(election_results) -> float:
     """
     Computes the partisan bias for the given ElectionResults.
     The partisan bias is defined as the number of districts with above-mean
     vote share by the first party divided by the total number of districts,
     minus 1/2.
+
+    :param election_results: An ElectionResults object
+    :type election_results: ElectionResults
+
+    :returns: The partisan bias for the given ElectionResults
+    :rtype: float
     """
     first_party = election_results.election.parties[0]
     party_shares = numpy.array(election_results.percents(first_party))
@@ -74,37 +112,34 @@ def partisan_bias(election_results):
     return (above_mean_districts / len(party_shares)) - 0.5
 
 
-def partisan_gini(election_results):
+def partisan_gini(election_results) -> float:
     """
     Computes the partisan Gini score for the given ElectionResults.
     The partisan Gini score is defined as the area between the seats-votes
     curve and its reflection about (.5, .5).
+
+    For more information on the computation, see Definition 1 in:
+    https://arxiv.org/pdf/2008.06930.pdf
+
+    :param election_results: An ElectionResults object
+    :type election_results: ElectionResults
+
+    :returns: The partisan Gini score for the given ElectionResults
+    :rtype: float
     """
     # For two parties, the Gini score is symmetric--it does not vary by party.
     party = election_results.election.parties[0]
 
-    # To find seats as a function of votes, we assume uniform partisan swing.
-    # That is, if the statewide popular vote share for a party swings by some
-    # delta, the vote share for that party swings by that delta in each
-    # district.
-    # We calculate the necessary delta to shift the district with the highest
-    # vote share for the party to a vote share of 0.5. This delta, subtracted
-    # from the original popular vote share, gives the minimum popular vote
-    # share that yields 1 seat to the party.
-    # We repeat this process for the district with the second-highest vote
-    # share, which gives the minimum popular vote share yielding 2 seats,
-    # and so on.
     overall_result = election_results.percent(party)
     race_results = sorted(election_results.percents(party), reverse=True)
     seats_votes = [overall_result - r + 0.5 for r in race_results]
 
     # Apply reflection of seats-votes curve about (.5, .5)
     reflected_sv = reversed([1 - s for s in seats_votes])
     # Calculate the unscaled, unsigned area between the seats-votes curve
-    # and its reflection. For each possible number of seats attained, we find
-    # the area of a rectangle of unit height, with a width determined by the
-    # horizontal distance between the curves at that number of seats.
+    # and its reflection.
     unscaled_area = sum(abs(s - r) for s, r in zip(seats_votes, reflected_sv))
+
     # We divide by area by the number of seats to obtain a partisan Gini score
     # between 0 and 1.
     return unscaled_area / len(race_results)