Where-clause based on attribute value in ArcPy

Question

I am trying to create a Python toolbox which will iterate through selected features and create a statistics table for each selected feature.

It works as follow:

Define parameters, including the input layers with selected features
Export selected features in new layer
Use searchCursor() to iterate through rows of the new layer
Get row value
Use row value in an expression to apply SelectLayerByAttribute()
Use selected feature to clip another input layer
Apply some other tools before compute statistics on the other input layer

fc = newLayer
field = "ID_reserve_castor"
cursor = arcpy.SearchCursor(fc)
row = cursor.next()

while row:

    value = row.getValue(field)

    # Process: Select Layer By Attribute (Select Layer By Attribute) (management)
    Reserve_castor_ExportFeature = arcpy.management.SelectLayerByAttribute(
        in_layer_or_view=fc,
        selection_type="NEW_SELECTION",
        where_clause='"ID_reserve_castor" = ' + value,
        invert_where_clause="")

    row = cursor.next()

The iteration and row.getValue() work, but not the SelectLayerByAttribute(). As the new layer is saved in a geodatabase, I used the exemple mentioned in esri technical support web page.

   #Identifies File Geodatabase
    if WPtype == "esriDataSourcesGDB.FileGDBWorkspaceFactory.1":
        #sample where clause for fgdb data
        WhereClause = '"ObjectID" = ' + value
        print WhereClause

What did I miss ?

Do not use arcpy.SearchCursor for anything. Instead usearcpy.da.SearchCursor. If you must manage the selection environment of a layer inside a cursor, make sure it doesn't corrupt the layer on which the cursor is already operating. Best practice is to never nest cursors. — Vince
– Vince, Commented Aug 15, 2022 at 23:16
@Vince You are my hero (Foo Fighters tune in my head). I ran into exactly that problem. it was "da" and suddenly I was one step further, where I hang for 3 hours agonizing about my stupid SQL. — Semo
– Semo, Commented Mar 15, 2024 at 13:35

dslamb · Accepted Answer · 2022-08-16 11:57:18Z

As stated by @Vince arcpy.SearchCursor is a legacy tool, and you should be looking at arcpy.da.SearchCursor (unless you only have access to a really old version of ArcMap).

Another suggestion is to hold off on writing the selected features to a new feature class, because you can use a selection directly in a search query and other tools. Writing to a file generally is a bottleneck in the code, and my preference is to save it to the last step.

An example to get you started:

selection = arcpy.management.SelectLayerByAttribute(fc,"NEW_SELECTION",
        where_clause='"ID_reserve_castor" = ' + value).getoutput(0)
with arcpy.da.SearchCursor(selection,["SHAPE@"]) as sc:
    for row in sc:
       feature = row[0]
       #you can clip or do something with the shape

If the field you are using in the where clause is string/text, you would need to place the value in single quotes. '"ID_reserve_castor" = ' + "'"+value+"'" (there is an easier way to do that, but I don't remember if it is available in Python 2.7).

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