Proper Exception Handling in Cursors

RogerDunnGIS

I am experienced programmer, and no stranger to exceptions, try blocks, and exception classes in programming languages.  But I continue to be proven wrong when it comes to exceptions being thrown during arcpy cursor operations.

I am working with a file geodatabase and whenever an exception is thrown during a cursor operation (whether Search, Insert, or Update), when I go back to my map, my layers don't draw; when I go back to my attribute table, it is empty and shows an error message sometimes.  Here is pseudo-code for the pattern I use.  Any feedback and explanations would be helpful.  Note that this pattern works just fine when no exceptions are thrown.

def sampleFunc(fc, flds, query, data):
    with arcpy.da.UpdateCursor(fc, flds, query) as uCursor:
        try:
            for row in uCursor:
                # some business logic here which works for most rows but
                # may throw an unhandled exception on a particular row.
                row[4] = aFunction(row[3])
                uCursor.updateRow(row)
        finally:
            del uCursor

 

In order to see my map and attribute tables again, I simply close down the project (with or without saving) and open it up again.

 

blake.terhune

In Python 3 (ArcGIS Pro) this is what I typically do, mostly to track where errors occur so I can more easily troubleshoot to data. Especially if this update cursor work is in a separate function than your main code.

 

with arcpy.da.UpdateCursor("myFeatureClass", ["someUniqueID", "Field1", "Field2"]) as uCursor:
    for uid, field1, field2 in uCursor:
        try:
            # Do some business logic.
            field2 = field1 * 100
            # Commit new values to table.
            # To throw an error with the cursor:
            # use a value that does not conform to your field type
            # or change the number of values in the row.
            uCursor.updateRow([uid, field1, field2, "thisWillError"])
        except Exception as e:
            raise Exception(f"Error updating row at someUniqueID {uid}") from e

 

If you want the most coverage possible, following the "suggestions" in the documentation, I suppose you could do something like this (though I never have).

with arcpy.da.UpdateCursor("AnchorPoints", ["someUniqueID", "Field1", "Field2"]) as uCursor:
    try:
        for uid, field1, field2 in uCursor:
            try:
                # Do some business logic.
                field2 = field1 * 100
                # Commit new values to table.
                uCursor.updateRow([uid, field1, field2])
            except Exception as e:
                raise Exception(f"Error updating row at someUniqueID {uid}") from e
    finally:
        del uCursor

As for having an incorrect field name, the cursor will fail to be created so there's no cursor object to delete. In that case, you would have to follow the suggestion in the documentation and have it in a separate function so the everything gets cleaned up with garbage collection when it goes out of scope on exit (regardless of an exception). That's pretty tedious though.

You might need to contact Esri support about the behavior your seeing when the cursor fails.