Why does CalculateField set values to <Null> when output extent is in a different coordinate system?

Question

I'm quite new to ArcGIS, and now I have to work on a script that goes through a list of feature classes to:

1. add a field to their attribute table and
2. fill that field with a number that I retrieve from somewhere else

When I execute the following statements in the ArcMap Python window or in a minimal standalone script, it works just fine:

table = "JS4A"
arcpy.AddField_management(table, "BK_Code", "LONG")
arcpy.CalculateField_management(table, "BK_Code", "1", "PYTHON_9.3", "")

I can then open the attribute table of that feature class in ArcMap and see that this field has been added and populated with all 1s, just as expected.

Now the weird thing is, when I run the exact same code in the complex production script, the field is added as well, but when I open the attribute table, all the values are <Null>.

I managed to track the problem down to the following line of code in my production script, that comes before the part with AddField and CalculateField:

env.extent = desc.extent

where desc is the return value of an arcpy.Describe() call to some other, unrelated shapefile. All CalculateField_management calls before that line will work fine, all calls after that line produce <Null>. When I replace the above line with a line that sets the output extent to a hard-coded value, it works.

I don't understand this at all. Is this an ArcGIS bug?

Unfortunately I can't omit the above line because it is required in the production script.

Answer 1

I just ran the test below using ArcGIS for Desktop 10.1 SP1 and it ran fine:

import arcpy

desc = arcpy.Describe("C:/temp/test6.shp")
arcpy.env.extent = desc.extent

table = "C:/temp/test.gdb/test2"
arcpy.AddField_management(table, "BK_Code", "LONG")
arcpy.CalculateField_management(table, "BK_Code", "1", "PYTHON_9.3", "")

Is "1" actually hardcoded in your script?

If not, and unless you have a selection on table, I can't see how you can be getting <Null> rather than 1 in your table from CalculateField_management.

Answer 2

I'm assuming you are running other tools as you shouldn't need to set the extents to add/calculate field. So my question would be why does it matter if calc field is before or after the line that is giving you difficulties?

That aside, it's hard to tell, but you might be having this issue...

When running a script from within ArcMap, you can drag and drop features from the table of contents, and it will run fine without the filename extension or setting the workspace as the current workspace is the default.

However, when running a standalone script, you must provide the extension (and optionally the workspace so that way all you need to specify is the file in the folder). If not specifying the workspace, you need to include the full path every time you access a file. For example:

arcpy.env.workspace = "D:/Data/"
arcpy.AddField_management("JS4A.shp", "Bk_code", "LONG")

Answer 3

The data type of the BK_Code field is LONG. Doesn't this mean a hard-coded 1 should not be wrapped in quotes? And if it is, and the system doesn't automatically cast the string "1" to a LONG, would that cause a NULL entry rather than an error message?