Using Select by Attribute within a for loop

Question

I am trying to iterate through a feature layer of counties, using each row as the selection expression for Select by Attribute. This selection will then be used to clip another layer. The goal is to break the initial layer into smaller pieces to aid in geoprocessing.

`import arcpy
counties = arcpy.MakeFeatureLayer_management(r"R:\Data\Base_Data\Administrative\Admin_Political_Bounds.gdb\HL_Political_Boundaries\HL_Counties_NJGIN", "counties")
workspace = r"R:\Data\Data_Development\HDC_Allocations\HDC_raw3.gdb"
with arcpy.da.SearchCursor(counties, ['SHAPE@AREA','COUNTY']) as cursor:
 for row in cursor:
 i=row[1]
 expression = '"COUNTY" = {0}'.format(i)
 county = arcpy.SelectLayerByAttribute_management(counties,"NEW_SELECTION", expression)
 outname = workspace + "_" + row[1]
 arcpy.Clip_analysis(HDC_raw, county, outname)
 arcpy.Delete_management(county)`

My problem is that I keep getting a syntax/invalid expression error on the Select Layer by Attribute line.

This is part of a much larger script that is run as a standalone script.

• SMiller: I'm confused - why are you doing a select by attribute while you're already in a search cursor for the same layer?
• Bera: counties is a result object, "counties" is the layer which you should be using in SelectLayerByAttributes
• SMiller: You can perform search cursor on either a feature class (e.g. r"R:\Data\Base_Data\Administrative\Admin_Political_Bounds.gdb\HL_Political_Boundaries\HL_Counties_NJGIN") or layer (e.g. "counties"), but SelectByAttributes requires the layer ("counties"). As BERA pointed out counties is a result object based on the creation of the featurelayer, but is not the actual layer name.
• radouxju: aren't you trying to do SplitByAttributes_analysis (counties,workspace, 'COUNTY') ? Deleting inside a searchcursor will be an issue after you solve the selectlayerbyattribute(). You should manage this with 2 loops (one search to get the county values, then one loop on the unique county values to select by attribute.
• JMVDA: @radouxju I don't need the counties as separate files, I need my target file split by county. I know there is a simple way of doing this, but I can't remember what it is.

Answer 1

Use AddFieldDelimiters and if COUNTY is a string/text field you are missing single quotes around the value:

expression = "{0} = '{1}'".format(arcpy.AddFieldDelimiters(datasource="counties", field="COUNTY"), i)

And counties is a result object, "counties" is the layer which you should be using in SelectLayerByAttributes

arcpy.SelectLayerByAttribute_management(in_layer_or_view="counties", selection_type="NEW_SELECTION", where_clause=expression)

Answer 2

The Split tool is the correct way of doing this. It takes the file that you want to split and splits it according to the features in a separate file. In this case, it takes my target layer and splits it by county, located in a separate layer.

http://desktop.arcgis.com/en/arcmap/10.3/tools/analysis-toolbox/split.htm