Creating where_clause in ArcPy gives ERROR 000358

Question

I would like to create a query in ArcPy without creating a temporary additional attribute first. Unfortunately I fail with the syntax for creating the expression. I want to check if a ratio of two existing attributes (RCT_WIDTH_MBG_Length_REWI and RCT_WIDTH_MBG_Width_REWID) falls below a certain value.

My latest code is:

    expression = '!RCT_WIDTH_MBG_Length_REWI! / !RCT_WIDTH_MBG_Width_REWID! > 3' 
    arcpy.SelectLayerByAttribute_management('Clean_LYR', 'NEW_SELECTION', expression)
    if int(arcpy.GetCount_management('Clean_LYR')[0]) > 0:
        print(str(int(arcpy.GetCount_management('Clean_LYR')[0])) + ' werden geloescht')
        arcpy.DeleteFeatures_management('Clean_LYR')

and during the script run I code the error message:

ExecuteError: ERROR 000358: Invalid expression

Answer 1

As commented by @Vince:

The "bang sandwich" only applies to Calculate Field (at the interface with Python). The SQL where_clause must be valid SQL (which means valid SQL identifiers -- just column names, no delimiter characters).

If your data source is a file geodatabase feature class or a shapefile, instead of using:

expression = '!RCT_WIDTH_MBG_Length_REWI! / !RCT_WIDTH_MBG_Width_REWID! > 3'

try using:

expression = 'RCT_WIDTH_MBG_Length_REWI / RCT_WIDTH_MBG_Width_REWID > 3'