fix: convert unspecified to serializable

Author: olavloite

google/cloud/sqlalchemy_spanner/requirements.py

@@ -102,7 +102,10 @@ def get_isolation_levels(self, _):
         Returns:
             dict: isolation levels description.
         """
-        return {"default": "SERIALIZABLE", "supported": ["SERIALIZABLE", "AUTOCOMMIT"]}
+        return {
+            "default": "SERIALIZABLE",
+            "supported": ["SERIALIZABLE", "REPEATABLE READ", "AUTOCOMMIT"],
+        }
 
     @property
     def precision_numerics_enotation_large(self):

google/cloud/sqlalchemy_spanner/sqlalchemy_spanner.py

@@ -813,6 +813,9 @@ def default_isolation_level(self):
 
     @default_isolation_level.setter
     def default_isolation_level(self, value):
+        unspecified = TransactionOptions.IsolationLevel.ISOLATION_LEVEL_UNSPECIFIED
+        if value == unspecified or value == unspecified.name:
+            value = "SERIALIZABLE"
         self._default_isolation_level = value
 
     def _check_unicode_returns(self, connection, additional_tests=None):