Determine the schema in load_table_from_dataframe based on dtypes.

This PR updates `load_table_from_dataframe` to automatically determine
the BigQuery schema based on the DataFrame's dtypes. If any field's type
cannot be determined, fallback to the logic in the pandas `to_parquet`
method.

bigquery/google/cloud/bigquery/_pandas_helpers.py

@@ -49,6 +49,21 @@
 
 _PROGRESS_INTERVAL = 0.2  # Maximum time between download status checks, in seconds.
 
+_PANDAS_DTYPE_TO_BQ = {
+    "bool": "BOOLEAN",
+    "datetime64[ns, UTC]": "TIMESTAMP",
+    "datetime64[ns]": "DATETIME",
+    "float32": "FLOAT",
+    "float64": "FLOAT",
+    "int8": "INTEGER",
+    "int16": "INTEGER",
+    "int32": "INTEGER",
+    "int64": "INTEGER",
+    "uint8": "INTEGER",
+    "uint16": "INTEGER",
+    "uint32": "INTEGER",
+}
+
 
 class _DownloadState(object):
     """Flag to indicate that a thread should exit early."""
@@ -172,6 +187,31 @@ def bq_to_arrow_array(series, bq_field):
     return pyarrow.array(series, type=arrow_type)
 
 
+def dataframe_to_bq_schema(dataframe):
+    """Convert a pandas DataFrame schema to a BigQuery schema.
+
+    TODO(GH#8140): Add bq_schema argument to allow overriding autodetected
+                   schema for a subset of columns.
+
+    Args:
+        dataframe (pandas.DataFrame):
+            DataFrame to convert to convert to Parquet file.
+
+    Returns:
+        Optional[Sequence[google.cloud.bigquery.schema.SchemaField]]:
+            The automatically determined schema. Returns None if the type of
+            any column cannot be determined.
+    """
+    bq_schema = []
+    for column, dtype in zip(dataframe.columns, dataframe.dtypes):
+        bq_type = _PANDAS_DTYPE_TO_BQ.get(dtype.name)
+        if not bq_type:
+            return None
+        bq_field = schema.SchemaField(column, bq_type)
+        bq_schema.append(bq_field)
+    return tuple(bq_schema)
+
+
 def dataframe_to_arrow(dataframe, bq_schema):
     """Convert pandas dataframe to Arrow table, using BigQuery schema.
 

bigquery/google/cloud/bigquery/client.py

@@ -21,6 +21,7 @@
 except ImportError:  # Python 2.7
     import collections as collections_abc
 
+import copy
 import functools
 import gzip
 import io
@@ -1520,11 +1521,19 @@ def load_table_from_dataframe(
 
         if job_config is None:
             job_config = job.LoadJobConfig()
+        else:
+            # Make a copy so that the job config isn't modified in-place.
+            job_config_properties = copy.deepcopy(job_config._properties)
+            job_config = job.LoadJobConfig()
+            job_config._properties = job_config_properties
         job_config.source_format = job.SourceFormat.PARQUET
 
         if location is None:
             location = self.location
 
+        if not job_config.schema:
+            job_config.schema = _pandas_helpers.dataframe_to_bq_schema(dataframe)
+
         tmpfd, tmppath = tempfile.mkstemp(suffix="_job_{}.parquet".format(job_id[:8]))
         os.close(tmpfd)
 

bigquery/tests/system.py

@@ -634,6 +634,100 @@ def test_load_table_from_local_avro_file_then_dump_table(self):
             sorted(row_tuples, key=by_wavelength), sorted(ROWS, key=by_wavelength)
         )
 
+    @unittest.skipIf(pandas is None, "Requires `pandas`")
+    @unittest.skipIf(pyarrow is None, "Requires `pyarrow`")
+    def test_load_table_from_dataframe_w_automatic_schema(self):
+        """Test that a DataFrame with dtypes that map well to BigQuery types
+        can be uploaded without specifying a schema.
+
+        https://github.com/googleapis/google-cloud-python/issues/9044
+        """
+        bool_col = pandas.Series([True, False, True], dtype="bool")
+        ts_col = pandas.Series(
+            [
+                datetime.datetime(2010, 1, 2, 3, 44, 50),
+                datetime.datetime(2011, 2, 3, 14, 50, 59),
+                datetime.datetime(2012, 3, 14, 15, 16),
+            ],
+            dtype="datetime64[ns]",
+        ).dt.tz_localize(pytz.utc)
+        dt_col = pandas.Series(
+            [
+                datetime.datetime(2010, 1, 2, 3, 44, 50),
+                datetime.datetime(2011, 2, 3, 14, 50, 59),
+                datetime.datetime(2012, 3, 14, 15, 16),
+            ],
+            dtype="datetime64[ns]",
+        )
+        float32_col = pandas.Series([1.0, 2.0, 3.0], dtype="float32")
+        float64_col = pandas.Series([4.0, 5.0, 6.0], dtype="float64")
+        int8_col = pandas.Series([-12, -11, -10], dtype="int8")
+        int16_col = pandas.Series([-9, -8, -7], dtype="int16")
+        int32_col = pandas.Series([-6, -5, -4], dtype="int32")
+        int64_col = pandas.Series([-3, -2, -1], dtype="int64")
+        uint8_col = pandas.Series([0, 1, 2], dtype="uint8")
+        uint16_col = pandas.Series([3, 4, 5], dtype="uint16")
+        uint32_col = pandas.Series([6, 7, 8], dtype="uint32")
+        dataframe = pandas.DataFrame(
+            {
+                "bool_col": bool_col,
+                "ts_col": ts_col,
+                "dt_col": dt_col,
+                "float32_col": float32_col,
+                "float64_col": float64_col,
+                "int8_col": int8_col,
+                "int16_col": int16_col,
+                "int32_col": int32_col,
+                "int64_col": int64_col,
+                "uint8_col": uint8_col,
+                "uint16_col": uint16_col,
+                "uint32_col": uint32_col,
+            },
+            columns=[
+                "bool_col",
+                "ts_col",
+                "dt_col",
+                "float32_col",
+                "float64_col",
+                "int8_col",
+                "int16_col",
+                "int32_col",
+                "int64_col",
+                "uint8_col",
+                "uint16_col",
+                "uint32_col",
+            ],
+        )
+
+        dataset_id = _make_dataset_id("bq_load_test")
+        self.temp_dataset(dataset_id)
+        table_id = "{}.{}.load_table_from_dataframe_w_automatic_schema".format(
+            Config.CLIENT.project, dataset_id
+        )
+
+        load_job = Config.CLIENT.load_table_from_dataframe(dataframe, table_id)
+        load_job.result()
+
+        table = Config.CLIENT.get_table(table_id)
+        self.assertEqual(
+            tuple(table.schema),
+            (
+                bigquery.SchemaField("bool_col", "BOOLEAN"),
+                bigquery.SchemaField("ts_col", "TIMESTAMP"),
+                bigquery.SchemaField("dt_col", "DATETIME"),
+                bigquery.SchemaField("float32_col", "FLOAT"),
+                bigquery.SchemaField("float64_col", "FLOAT"),
+                bigquery.SchemaField("int8_col", "INTEGER"),
+                bigquery.SchemaField("int16_col", "INTEGER"),
+                bigquery.SchemaField("int32_col", "INTEGER"),
+                bigquery.SchemaField("int64_col", "INTEGER"),
+                bigquery.SchemaField("uint8_col", "INTEGER"),
+                bigquery.SchemaField("uint16_col", "INTEGER"),
+                bigquery.SchemaField("uint32_col", "INTEGER"),
+            ),
+        )
+        self.assertEqual(table.num_rows, 3)
+
     @unittest.skipIf(pandas is None, "Requires `pandas`")
     @unittest.skipIf(pyarrow is None, "Requires `pyarrow`")
     def test_load_table_from_dataframe_w_nulls(self):