Strip PostgreSQL type cast specifiers when comparing computed defaults

alembic/autogenerate/compare/server_defaults.py

@@ -21,6 +21,7 @@
 if TYPE_CHECKING:
     from sqlalchemy.sql.elements import quoted_name
     from sqlalchemy.sql.schema import Column
+    from sqlalchemy.engine import Dialect
 
     from ...autogenerate.api import AutogenContext
     from ...operations.ops import AlterColumnOp
@@ -48,14 +49,20 @@ def _render_server_default_for_compare(
         return None
 
 
-def _normalize_computed_default(sqltext: str) -> str:
+def _normalize_computed_default(sqltext: str, dialect: Dialect) -> str:
     """we want to warn if a computed sql expression has changed.  however
     we don't want false positives and the warning is not that critical.
     so filter out most forms of variability from the SQL text.
 
     """
 
-    return re.sub(r"[ \(\)'\"`\[\]\t\r\n]", "", sqltext).lower()
+    normalized_sqltext = re.sub(r"[ \(\)'\"`\[\]\t\r\n]", "", sqltext).lower()
+    if dialect.name == "postgresql":
+        # strip postgresql type cast specifiers, e.g. ``::regconfig``,
+        # ``::text``, which can appear inconsistently on the reflected side
+        # only and cause false-positive warnings.
+        normalized_sqltext = re.sub(r"::\w+", "", normalized_sqltext)
+    return normalized_sqltext
 
 
 def _compare_computed_default(
@@ -94,7 +101,7 @@ def _compare_computed_default(
     # get a minimal comparison just to emit a warning.
 
     rendered_metadata_default = _normalize_computed_default(
-        rendered_metadata_default
+        rendered_metadata_default, autogen_context.dialect
     )
 
     if isinstance(conn_col.server_default, sa_schema.Computed):
@@ -105,7 +112,7 @@ def _compare_computed_default(
             )
         )
         rendered_conn_default = _normalize_computed_default(
-            rendered_conn_default
+            rendered_conn_default, autogen_context.dialect
         )
     else:
         rendered_conn_default = ""

docs/build/unreleased/1462.rst

@@ -0,0 +1,10 @@
+.. change::
+    :tags: bug, autogenerate, postgresql
+    :tickets: 1462
+
+    Fixed false-positive "Computed default cannot be modified" warning emitted
+    during autogenerate when comparing PostgreSQL ``Computed`` columns whose
+    reflected SQL expression includes type cast specifiers such as
+    ``::regconfig`` or ``::text``.  These cast specifiers are now stripped
+    before comparison so that semantically equivalent expressions no longer
+    trigger the warning.

tests/test_postgresql.py

@@ -21,6 +21,7 @@
 from sqlalchemy import text
 from sqlalchemy import types
 from sqlalchemy import UniqueConstraint
+from sqlalchemy.dialects import postgresql
 from sqlalchemy.dialects.postgresql import ARRAY
 from sqlalchemy.dialects.postgresql import BYTEA
 from sqlalchemy.dialects.postgresql import ExcludeConstraint
@@ -72,6 +73,9 @@
     from alembic.autogenerate.compare.server_defaults import (
         _dialect_impl_compare_server_default as _compare_server_default,
     )
+    from alembic.autogenerate.compare.server_defaults import (
+        _normalize_computed_default,
+    )
 
 
 class PostgresqlOpTest(TestBase):
@@ -395,6 +399,22 @@ def test_alter_column_computed_not_supported(self, sd, esd):
             existing_server_default=esd(),
         )
 
+    def test_normalize_computed_default_strips_type_casts(self):
+        # type cast specifiers such as ``::regconfig`` only appear on the
+        # reflected (connection) side and would otherwise cause a
+        # false-positive "cannot be modified" warning.  issue #1462
+        dialect = postgresql.dialect()
+        eq_(
+            _normalize_computed_default(
+                "setweight(to_tsvector('english', title), 'a')", dialect
+            ),
+            _normalize_computed_default(
+                "setweight(to_tsvector('english'::regconfig, "
+                "title::text), 'a'::\"char\")",
+                dialect,
+            ),
+        )
+
     @combinations(
         ({}, None),
         (dict(always=True), None),