Working with Partitions

This document walks you through the most common ways that you might work with a GerryChain Partition object.

from gerrychain import Partition, Graph
from gerrychain.updaters import cut_edges

We’ll use our Pennsylvania VTD json to create the graph we’ll use in these examples.

graph = Graph.from_json("./PA_VTDs.json")

Creating a partition

Here is how you can create a Partition:

partition = Partition(graph, "2011_PLA_1", {"cut_edges": cut_edges})

The Partition class takes three arguments to create a Partition:

A graph.
An assignment of nodes to districts. This can be the string name of a node attribute (shapefile column) that holds each node’s district assignment, or a dictionary mapping each node ID to its assigned district ID.
A dictionary of updaters.

This creates a partition of the graph object we created above from the Pennsylvania shapefile. The partition is defined by the "CD_2011" column from our shapefile’s attribute table.

`partition.graph`: the underlying graph

You can access the partition’s underlying Graph as partition.graph. This contains no information about the partition—it will be the same graph object that you passed in to Partition() when you created the partition instance.

partition.graph is a gerrychain.Graph object. It is based on the NetworkX Graph object, so any functions (e.g. connected_components) you can find in the NetworkX documentation will be compatible.

partition.graph

prints something like:

<gerrychain.graph.graph.FrozenGraph object at 0x7fb0a93bbfa0>

Now we have a graph of Pennsylvania’s VTDs, with all of the data from our shapefile’s attribute table attached to the graph as node attributes. We can see the data that a node has like this:

partition.graph.nodes[0]

which will output:

{'boundary_node': False,
'area': 0.0063017857514999324,
'STATEFP10': '42',
'COUNTYFP10': '039',
'VTDST10': '60',
'GEOID10': '42039060',
'VTDI10': 'A',
'NAME10': 'CAMBRIDGE SPRINGS Voting District',
'NAMELSAD10': 'CAMBRIDGE SPRINGS Voting District',
'LSAD10': '00',
'MTFCC10': 'G5240',
'FUNCSTAT10': 'N',
'ALAND10': 2258229,
'AWATER10': 0,
'INTPTLAT10': '+41.8018353',
'INTPTLON10': '-080.0596566',
'ATG12D': 0.0,
'ATG12R': 0.0,
'GOV10D': 0.0,
'GOV10R': 0.0,
'PRES12D': 0.0,
'PRES12O': 0.0,
'PRES12R': 0.0,
'SEN10D': 0.0,
'SEN10R': 0.0,
'T16ATGD': 0.0,
'T16ATGR': 0.0,
'T16PRESD': 0,
'T16PRESOTH': 0.0,
'T16PRESR': 0,
'T16SEND': 0,
'T16SENR': 0,
'USS12D': 0.0,
'USS12R': 0.0,
'GOV': 3,
'TS': 5,
'HISP_POP': 0,
'TOT_POP': 0,
'WHITE_POP': 0,
'BLACK_POP': 0,
'NATIVE_POP': 0,
'ASIAN_POP': 0,
'F2014GOVD': 1,
'F2014GOVR': 1,
'2011_PLA_1': 3,
'REMEDIAL_P': 14,
'538CPCT__1': 3,
'538DEM_PL': 3,
'538GOP_PL': 3,
'8THGRADE_1': 1}

`partition.assignment`: assign nodes to parts

partition.assignment gives you a mapping from node IDs to part IDs (“part” is our generic word for “district”). It is a custom data structure but you can use it just like a dictionary. So the code:

first_ten_nodes = list(partition.graph.nodes)[:10]
for node in first_ten_nodes:
    print(partition.assignment[node])

will output:

`partition.parts`: the nodes in each part

partition.parts gives you a mapping from each part ID to the set of nodes that belong to that part. This is the “opposite” mapping of assignment.

As an example, let’s print out the number of nodes in each part:

for part in partition.parts:
    number_of_nodes = len(partition.parts[part])
    print(f"Part {part} has {number_of_nodes} nodes")

This will give us:

Part 3 has 469 nodes
Part 10 has 462 nodes
Part 9 has 515 nodes
Part 5 has 513 nodes
Part 15 has 317 nodes
Part 6 has 310 nodes
Part 11 has 440 nodes
Part 8 has 337 nodes
Part 4 has 271 nodes
Part 18 has 591 nodes
Part 12 has 597 nodes
Part 17 has 412 nodes
Part 7 has 404 nodes
Part 16 has 322 nodes
Part 14 has 867 nodes
Part 13 has 548 nodes
Part 2 has 828 nodes
Part 1 has 718 nodes

`partition.subgraphs`: the subgraphs of each part

For each part of our partition, we can look at the _subgraph_ that it defines. That is, we can look at the graph made up of all the nodes in a certain part and all the edges between those nodes.

partition.subgraphs gives us a mapping (like a dictionary) from part IDs to their subgraphs. These subgraphs are NetworkX Subgraph objects, and work exactly like our main graph object—nodes, edges, and node attributes all work the same way.

for part, subgraph in partition.subgraphs.items():
    number_of_edges = len(subgraph.edges)
    print(f"Part {part} has {number_of_edges} edges")

This will output:

Part 3 has 1195 edges
Part 10 has 1183 edges
Part 9 has 1314 edges
Part 5 has 1349 edges
Part 15 has 824 edges
Part 6 has 745 edges
Part 11 has 1134 edges
Part 8 has 881 edges
Part 4 has 693 edges
Part 18 has 1575 edges
Part 12 has 1559 edges
Part 17 has 1015 edges
Part 7 has 930 edges
Part 16 has 825 edges
Part 14 has 2344 edges
Part 13 has 1362 edges
Part 2 has 2159 edges
Part 1 has 1780 edges

Let’s use NetworkX’s diameter function to compute the diameter of each part subgraph. (The _diameter_ of a graph is the length of the longest path in the set of shortest paths between any two nodes in the given graph, but you don’t have to know that!)

import networkx
for part, subgraph in partition.subgraphs.items():
    diameter = networkx.diameter(subgraph)
    print(f"Part {part} has diameter {diameter}")

This outputs:

Part 3 has diameter 40
Part 10 has diameter 40
Part 9 has diameter 40
Part 5 has diameter 29
Part 15 has diameter 28
Part 6 has diameter 32
Part 11 has diameter 31
Part 8 has diameter 24
Part 4 has diameter 19
Part 18 has diameter 28
Part 12 has diameter 35
Part 17 has diameter 35
Part 7 has diameter 38
Part 16 has diameter 38
Part 14 has diameter 38
Part 13 has diameter 30
Part 2 has diameter 28
Part 1 has diameter 50

Outputs of updaters

The other main way we can extract information from partition is through the updaters that we configured when we created it. We gave partition just one updater, cut_edges. This is the set of edges that go between nodes that are in _different_ parts of the partition. We should note that the updaters for our partition are both an item and an attribute of the partition, so we can access them with:

len(partition["cut_edges"])

which outputs:

or

len(partition.cut_edges)

which also outputs:

So if we wanted to print out the proportion of cut edges present within our graph, we might write:

proportion_of_cut_edges = len(partition.cut_edges) / len(partition.graph.edges)
print("Proportion of edges that are cut:")
print(proportion_of_cut_edges)

this will output:

Proportion of edges that are cut:
0.09358649120025368

Working with Partitions

Creating a partition

partition.graph: the underlying graph

partition.assignment: assign nodes to parts

partition.parts: the nodes in each part

partition.subgraphs: the subgraphs of each part

Outputs of updaters

`partition.graph`: the underlying graph

`partition.assignment`: assign nodes to parts

`partition.parts`: the nodes in each part

`partition.subgraphs`: the subgraphs of each part