ArcPy adds erroneous interior ring when writing adjacent, multipart geometry?

Question

I've run into an odd issue when using Arcpy to write multipart geometry to a shapefile. I am using an InsertCursor to create a multipart feature from a list of parts each with a list of vertex pairs. I understand that when this feature is created, adjacent multiple parts are automatically "dissolved" into a single part. But for some reason, this is creating an interior ring, even though I did not include a Null arcpy.point() in the array as is usually required to add interior rings. Here's a visualization:

Does anyone have any idea why this is happening and/or how to overcome the issue?

For reference, here's my code:

import arcpy

arcpy.CreateFeatureclass_management(r"C:\temp", "test.shp", "POLYGON")
OutputCursor = arcpy.InsertCursor(r"C:\temp\test.shp")

# List of parts, each with list of vertex pairs
ListOfParts = []
ListOfParts.append([[0,1],[1,1],[1,0],[0,0],[0,1]])
ListOfParts.append([[0,2],[1,2],[1,1],[0,1],[0,2]])
ListOfParts.append([[0,3],[1,3],[1,2],[0,2],[0,3]])
ListOfParts.append([[1,1],[2,1],[2,0],[1,0],[1,1]])
ListOfParts.append([[1,2],[2,2],[2,1],[1,1],[1,2]])
ListOfParts.append([[1,3],[2,3],[2,2],[1,2],[1,3]])
ListOfParts.append([[2,1],[3,1],[3,0],[2,0],[2,1]])
ListOfParts.append([[2,2],[3,2],[3,1],[2,1],[2,2]])
ListOfParts.append([[2,3],[3,3],[3,2],[2,2],[2,3]])

# Array of parts to be passed to newRow()
ArrayOfParts = arcpy.Array()

# Add parts to array
for Part in ListOfParts:
    ArrayOfVertices = arcpy.Array()
    for Vertex in Part:
        ArrayOfVertices.add(arcpy.Point(Vertex[0],Vertex[1]))
    ArrayOfParts.add(ArrayOfVertices)
    ArrayOfVertices.removeAll()

# Output new feature
OutputFeature = OutputCursor.newRow()
OutputFeature.shape = ArrayOfParts
OutputCursor.insertRow(OutputFeature)

Answer 1

You've defined points in the interior of the polygon to include in the shape. This would create the output you've been getting rather than the output you want. The program treats each point put into the polygon definition as defining an vertex of the polygon, so if you include all vertices in the polygon definition it will return a polygon with an edge between each vertex. To eliminate the ring you would need to separately create a polygon for each box in the grid and then dissolve the polygons together.

Alternatively, your code above could be edited as follows to include only the exterior points in the square:

import arcpy

arcpy.CreateFeatureclass_management(r"C:\temp", "test.shp", "POLYGON")
OutputCursor = arcpy.InsertCursor(r"C:\temp\test.shp")

# List of parts, each with list of vertex pairs
ListOfParts = []
ListOfParts.append([[0,3],[3,3],[3,0],[0,0],[0,3]])

# Array of parts to be passed to newRow()
ArrayOfParts = arcpy.Array()

# Add parts to array
for Part in ListOfParts:
    ArrayOfVertices = arcpy.Array()
    for Vertex in Part:
        ArrayOfVertices.add(arcpy.Point(Vertex[0],Vertex[1]))
    ArrayOfParts.add(ArrayOfVertices)
    ArrayOfVertices.removeAll()

# Output new feature
OutputFeature = OutputCursor.newRow()
OutputFeature.shape = ArrayOfParts
OutputCursor.insertRow(OutputFeature)