I have a geodatabase featureclass that I need to add a field to. I can open this file in ArcMap and make all the edits I need, adding fields, calculating those fields, etc., but when I try to replicate the results in Python using the Add Field tool, I get this error message:

ERROR 000499: table is not editable

Failed to execute (AddField).

The file is stored in a file geodatabase, and I've made sure no other Arc programs are accessing it, so locks shouldn't be an issue as far as I can tell.

Why would I be able to edit the file in one program but not the other? How can I fix this?

It seems like it also works to bring up the python window within ArcMap/Catalog. The problem only exists when I use the Python Shell outside of Arc.

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