Why can't I use SelectLayerByAttribute_management while a cursor is set

Question

I've been having a problem with arcpy in that the simple line:

arcpy.SelectLayerByAttribute_management ("TEMP_LINES")

produces this response: Runtime error : ERROR 000358: Invalid expression Failed to execute (SelectLayerByAttribute).

I've worked out that this error only occurs when a cursor is set, like this:

lineCursor = arcpy.UpdateCursor("TEMP_LINES","","","combined_i")

Can I not use select by attribute and the cursor or do I have one of them set up wrong?

EDIT: whole code:

arcpy.MakeFeatureLayer_management (Lines,"TEMP_LINES")
lineCursor = arcpy.UpdateCursor("TEMP_LINES","","","combined_i")
for row in lineCursor:
 arcpy.SelectLayerByAttribute_management ("TEMP_LINES")
 arcpy.SelectLayerByLocation_management (Polygons, "INTERSECT", "TEMP_LINES")
 arcpy.FeatureClassToFeatureClass_conversion (Polygons, "temp_polys")
 polyCursor = arcpy.SearchCursor("temp_polys")
 blocks_list = []
 for row in cursor:
 blocks_list.append(row.getValue("BLOCK"))
 blocks_str = ', '.join(blocks_list)
 lineCursor.BLOCK = blocks_str 

• Alex Tereshenkov: You don't provide a valid expression for the Select Layer by Attribute GP tool. You only provide the layer name which is not enough.
• whatahitson: without the cursor on this selects all. It doesnt work with any expression with the cursor on
• Alex Tereshenkov: Can you provide the whole chunk of code you execute please? I am a bit confused in which sequence you have those rows.
• whatahitson: @AlexTereshenkov I've edited to include
• geogeek: i've tested your code succefully, without any problems, it seems that the problem is not located in SelectLayerByAttribute_management

Answer 1

As stated by others you have not provided a selection criteria for arcpy.SelectLayerByAttribute_management(). Additionally using arcpy.SelectLayerByAttribute_management() within an Update Cursor is only going to apply to a single row and does not really make sense to do.

If I understand what you are trying to accomplish, I think you need to rework your logic in this script. Perhaps run the arcpy.SelectLayerByLocation_management() previous to creating your cursors and use a where_clause as an argument to your arcpy.UpdateCursor to limit the number of records that you are searching and updating against.

Answer 2

The SelectLayerByAttribute_management tool in your script is trying to select all features within the feature layer, whereas the cursor accesses one row at a time within the feature. If you would still like to use the tool within the cursor then there is a bit of an adjustment needed. I should make mention that this adjustment assumes the field "combined_i" is not unique.

lineCursor = arcpy.UpdateCursor("TEMP_LINES","","","combined_i; unique_field")
cols = arcpy.ListFields("TEMP_LINES")
for row in lineCursor:
 for col in cols:
 if col.name == "unique_field":
 objstr = row.getValue(col.name)
 arcpy.SelectLayerByAttribute_management("TEMP_LINES", "NEW_SELECTION", ("%s = %s" % (col.name, objstr)))
 .
 .
 .

Allow me to explain the additional code in more detail:

cols = arcpy.ListFields("TEMP_LINES")
 for col in cols:
 if col.name == "unique_field":
 objstr = row.getValue(col.name)

First, list all fields in "TEMP_LINES", then loop through the fields and find a field named "unique_field". If true then set objstr equal to the value of "unique_field" in the current row.

 arcpy.SelectLayerByAttribute_management ("TEMP_LINES", "NEW_SELECTION", ("%s = %s" % (col.name, objstr)))

Instead of trying to select all features in "TEMP_LINES" this will only select the feature with the unique value in "unique_field" at the current row.