Add option to disable cut_edges updater with use_cut_edges flag (#375)

Author: InnovativeInventor

gerrychain/partition/partition.py

@@ -16,7 +16,6 @@ class Partition:
     :ivar dict parts: Maps district IDs to the set of nodes in that district.
     :ivar dict subgraphs: Maps district IDs to the induced subgraph of that district.
     """
-    default_updaters = {"cut_edges": cut_edges}
     __slots__ = (
         'graph',
         'subgraphs',
@@ -30,26 +29,28 @@ class Partition:
     )
 
     def __init__(
-        self, graph=None, assignment=None, updaters=None, parent=None, flips=None
+        self, graph=None, assignment=None, updaters=None, parent=None, flips=None,
+        use_cut_edges=True
     ):
         """
         :param graph: Underlying graph.
         :param assignment: Dictionary assigning nodes to districts.
         :param updaters: Dictionary of functions to track data about the partition.
             The keys are stored as attributes on the partition class,
             which the functions compute.
+        :param use_cut_edges: If `False`, do not include `cut_edges` updater by default
+            and do not calculate edge flows.
         """
         if parent is None:
-            self._first_time(graph, assignment, updaters)
+            self._first_time(graph, assignment, updaters, use_cut_edges)
         else:
             self._from_parent(parent, flips)
 
         self._cache = dict()
         self.subgraphs = SubgraphView(self.graph, self.parts)
 
-    def _first_time(self, graph, assignment, updaters):
+    def _first_time(self, graph, assignment, updaters, use_cut_edges):
         self.graph = graph
-
         self.assignment = get_assignment(assignment, graph)
 
         if set(self.assignment) != set(graph):
@@ -58,7 +59,11 @@ def _first_time(self, graph, assignment, updaters):
         if updaters is None:
             updaters = dict()
 
-        self.updaters = self.default_updaters.copy()
+        if use_cut_edges:
+            self.updaters = {"cut_edges": cut_edges}
+        else:
+            self.updaters = {}
+
         self.updaters.update(updaters)
 
         self.parent = None
@@ -77,7 +82,9 @@ def _from_parent(self, parent, flips):
         self.updaters = parent.updaters
 
         self.flows = flows_from_changes(parent.assignment, flips)
-        self.edge_flows = compute_edge_flows(self)
+
+        if "cut_edges" in self.updaters:
+            self.edge_flows = compute_edge_flows(self)
 
     def __repr__(self):
         number_of_parts = len(self)

tests/partition/test_partition.py

@@ -56,7 +56,7 @@ def example_geographic_partition():
 
 def test_geographic_partition_can_be_instantiated(example_geographic_partition):
     partition = example_geographic_partition
-    assert partition.updaters == GeographicPartition.default_updaters
+    assert isinstance(partition, GeographicPartition)
 
 
 def test_Partition_parts_is_a_dictionary_of_parts_to_nodes(example_partition):
@@ -144,11 +144,9 @@ def test_repr(example_partition):
 
 def test_partition_has_default_updaters(example_partition):
     partition = example_partition
-    default_updaters = partition.default_updaters
     should_have_updaters = {"cut_edges": cut_edges}
 
     for updater in should_have_updaters:
-        assert default_updaters.get(updater, None) is not None
         assert should_have_updaters[updater](partition) == partition[updater]