feat: expose DataFrame.bqclient to assist in integrations (#519)

* feat: expose `DataFrame.session` to assist in integrations

* add code sample for sharing when the service account is only known on the backend

* expose bqclient instead

* start a DF construction sample

* finish read sample

Author: tswast

bigframes/dataframe.py

@@ -306,6 +306,11 @@ def empty(self) -> bool:
     def values(self) -> numpy.ndarray:
         return self.to_numpy()
 
+    @property
+    def bqclient(self) -> bigframes.Session:
+        """BigQuery REST API Client the DataFrame uses for operations."""
+        return self._session.bqclient
+
     @property
     def _session(self) -> bigframes.Session:
         return self._get_block().expr.session

notebooks/dataframes/dataframe.ipynb

@@ -1,5 +1,27 @@
 {
  "cells": [
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "id": "eeec3428",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "# Copyright 2023 Google LLC\n",
+    "#\n",
+    "# Licensed under the Apache License, Version 2.0 (the \"License\");\n",
+    "# you may not use this file except in compliance with the License.\n",
+    "# You may obtain a copy of the License at\n",
+    "#\n",
+    "#     https://www.apache.org/licenses/LICENSE-2.0\n",
+    "#\n",
+    "# Unless required by applicable law or agreed to in writing, software\n",
+    "# distributed under the License is distributed on an \"AS IS\" BASIS,\n",
+    "# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.\n",
+    "# See the License for the specific language governing permissions and\n",
+    "# limitations under the License."
+   ]
+  },
   {
    "attachments": {},
    "cell_type": "markdown",