- existing_server_default is not used to drop the previous

schema type constraint; existing_type is.  I haven't checked to
see if this changed at some point but that's what the source code
right now says.

diff --git a/alembic/operations/ops.py b/alembic/operations/ops.py
index 29f749e516f45836303eac0cf410059965a1f2d8..839c2da296fba301c5df66323f7e649e85efe796 100644 (file)
--- a/alembic/operations/ops.py
+++ b/alembic/operations/ops.py
@@ -1345,9 +1345,9 @@ class AlterColumnOp(AlterTableOp):
         and  :class:`~sqlalchemy.types.Enum` may also
         add or drop constraints which accompany those
         types on backends that don't support them natively.
-        The ``existing_server_default`` argument is
-        used in this case as well to remove a previous
-        constraint.
+        The ``existing_type`` argument is
+        used in this case to identify and remove a previous
+        constraint that was bound to the type object.
 
         :param table_name: string name of the target table.
         :param column_name: string name of the target column,