docs: add bigframes default connection warning (#2471)

fixes b/471256706

Author: GarrettWu

bigframes/bigquery/_operations/ai.py

@@ -1018,7 +1018,7 @@ def _convert_series(
 
 def_resolve_connection_id(series: series.Series, connection_id: str|None):
 returnclients.get_canonical_bq_connection_id(
-connection_idorseries._session._bq_connection,
+connection_idorseries._session.bq_connection,
 series._session._project,
 series._session._location,
  )

bigframes/functions/_function_session.py

@@ -162,7 +162,7 @@ def _resolve_bigquery_connection_id(
  ) ->str:
 """Resolves BigQuery connection id."""
 ifnotbigquery_connection:
-bigquery_connection=session._bq_connection# type: ignore
+bigquery_connection=session.bq_connection# type: ignore
 
 bigquery_connection=clients.get_canonical_bq_connection_id(
 bigquery_connection,

bigframes/operations/blob.py

@@ -311,7 +311,7 @@ def _resolve_connection(self, connection: Optional[str] = None) -> str:
  Raises:
  ValueError: If the connection cannot be resolved to a valid string.
  """
-connection=connectionorself._data._block.session._bq_connection
+connection=connectionorself._data._block.session.bq_connection
 returnclients.get_canonical_bq_connection_id(
 connection,
 default_project=self._data._block.session._project,

bigframes/session/__init__.py

@@ -374,6 +374,16 @@ def _allows_ambiguity(self) -> bool:
 def_anonymous_dataset(self):
 returnself._anon_dataset_manager.dataset
 
+@property
+defbq_connection(self) ->str:
+msg=bfe.format_message(
+f"""You are using the BigFrames session default connection: {self._bq_connection},
+ which can be different from the BigQuery project default connection.
+ This default connection may change in the future."""
+ )
+warnings.warn(msg, category=FutureWarning)
+returnself._bq_connection
+
 def__hash__(self):
 # Stable hash needed to use in expression tree
 returnhash(str(self._session_id))
@@ -2253,7 +2263,7 @@ def _create_bq_connection(
  ) ->str:
 """Create the connection with the session settings and try to attach iam role to the connection SA.
  If any of project, location or connection isn't specified, use the session defaults. Returns fully-qualified connection name."""
-connection=self._bq_connectionifnotconnectionelseconnection
+connection=self.bq_connectionifnotconnectionelseconnection
 connection=bigframes.clients.get_canonical_bq_connection_id(
 connection_id=connection,
 default_project=self._project,