fix(postgresql): identity column not added when existing_server_default omitted

alembic/ddl/postgresql.py

@@ -569,8 +569,9 @@ def visit_identity_column(
         # drop identity
         text += "DROP IDENTITY"
         return text
-    elif element.existing_server_default is None:
-        # add identity options
+    elif element.existing_server_default in (None, False):
+        # no known existing default (False is the "not specified"
+        # sentinel used by Operations.alter_column) - add identity options
         text += "ADD "
         text += compiler.visit_identity_column(element.default)
         return text

docs/build/unreleased/1504.rst

@@ -0,0 +1,13 @@
+.. change::
+    :tags: bug, postgresql
+    :tickets: 1504
+
+    Fixed bug where :meth:`.Operations.alter_column` against a PostgreSQL
+    identity column would render an incomplete, invalid ``ALTER TABLE ...
+    ALTER COLUMN`` statement with no clause at all when
+    ``existing_server_default`` was left at its default of not being
+    specified. The identity-column DDL compiler only recognized an explicit
+    ``None`` as "no existing default", not the sentinel value used
+    internally by :meth:`.Operations.alter_column` when the argument is
+    omitted, so the "add identity" branch was skipped and no options were
+    ever rendered.

tests/test_postgresql.py

@@ -415,21 +415,31 @@ def test_add_column_identity(self, kw, text):
         )
 
     @combinations(
-        ({}, None),
-        (dict(always=True), None),
+        ({}, None, True),
+        ({}, None, False),
+        (dict(always=True), None, True),
         (
             dict(start=3, increment=33, maxvalue=99, cycle=True),
             "INCREMENT BY 33 START WITH 3 MAXVALUE 99 CYCLE",
+            True,
         ),
     )
-    def test_add_identity_to_column(self, kw, text):
+    def test_add_identity_to_column(
+        self, kw, text, pass_existing_server_default
+    ):
         context = op_fixture("postgresql")
-        op.alter_column(
-            "t1",
-            "some_column",
-            server_default=Identity(**kw),
-            existing_server_default=None,
-        )
+        alter_column_kw = {
+            "server_default": Identity(**kw),
+        }
+        if pass_existing_server_default:
+            alter_column_kw["existing_server_default"] = None
+        # else: leave existing_server_default at its default, which is the
+        # ``False`` "not specified" sentinel, not ``None``. It must be
+        # treated the same as an explicit ``None``, or the identity is
+        # never actually added.
+        # Regression test for
+        # https://github.com/sqlalchemy/alembic/issues/1504
+        op.alter_column("t1", "some_column", **alter_column_kw)
         qualification = "ALWAYS" if kw.get("always", False) else "BY DEFAULT"
         options = " (%s)" % text if text else ""
         context.assert_(