How to iterate through a list, make attribute & spatial selections, and populate fields using cursors with ArcPy

Question

I have two shapefiles, one is points and the other polygons and they share a common attribute field Field_Name that is only populated in the points shapefile. I want to create a Python/ArcPy script that will accomplish the following:

Create a unique list from the values in the populated field in the points shapefile.
Iterate through the unique list, for each value:
Select the points features by attribute based on name in the unique list.
Select polygon features by location using previously selected points that are contained within the polygons.
Populate the null fields in the polygon shapefile with the values from the selected points features (same as the value from unique list).

This is what I have so far, anyone know how I can get it to work?

    import arcpy
    arcpy.env.workspace = r"C:\Users\*****\Desktop\GIS_test\shapefiles"

    Point = r'C:\Users\*****\Desktop\GIS_test\shapefiles\Point.shp'
    field = 'Field_Name'
    Polygons = r'C:\Users\*****\Desktop\GIS_test\shapefiles\Polygons.shp'

    # Use SearchCursor to return a unique set of values in the specified field    and create set.
    values = [row[0] for row in arcpy.da.SearchCursor(Point, field)]
    uniqueValues = set(values)

    # Convert the set to a list.
    unique_list = (list(uniqueValues)) 

    #Loop through list of unique values.
    for x in unique_list: 
        #SelectLayerByAttribute(in_layer_or_view, {selection_type}, {where_clause}, {invert_where_clause})
        arcpy.SelectLayerByAttribute_management(Point, 'NEW_SELECTION', [Field_Name] = '{0}'.format(x))

        #SelectLayerByLocation_management (in_layer, {overlap_type}, {select_features}, {search_distance},      
        #{selection_type},{invert_spatial_relationship})
        arcpy.SelectLayerByLocation_management(Polygons, 'CONTAINS', Point)

        #Update Field_Name attribute value in Polygon attribute table with name in unique list from Point.shp
        with arcpy.da.UpdateCursor(Polygons, 'Field_Name') as cursor:
            for row in cursor:
                row[0] = x
                cursor.updateRow(row)

The script is executing as far as line 18 (first select statement).

score 0 · Answer 1 · answered Aug 01 '19 at 16:44

It seems to me that what you're trying to do is a spatial join.

Spatial join from Arcmap documentation: Joins attributes from one feature to another based on the spatial relationship.

The documentation will explain far better than I how to use it.

Note 1: If multiple points with differents values are within a single polygon, the polygon will only have the fields of only one point.

Note 2: The spatial join function join all the fields from the join_features to the target_feature. So if you only want one column to be added, you have to hide the other fields. Here is an example how to hide fields (must create a new layer):

input_layer = 'your_layer_name'
desired_fields =  #['field1', 'field2', ...] 
field_info = arcpy.Describe(input_layer).fieldInfo
for i in range(field_info.count):
    if field_info.getfieldname(i) not in desired_fields:
        field_info.setvisible(i, 'HIDDEN')
arcpy.MakeFeatureLayer_management(input_layer, 'new_layer_name', '', '', field_info)

How to iterate through a list, make attribute & spatial selections, and populate fields using cursors with ArcPy

1 Answers1