Source code for gerrychain.metrics.compactness

import math
from typing import Dict


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


# 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(
            partition["area"][part], partition["perimeter"][part]
        )
        for part in partition.parts
    }