Add docstrings and typing to db_engine_specs and sql_parse (#8058)

* Add typing to db_engine_specs

* Add more type annotations and docstrings

* Add docstrings and typing to sql_parse and db_engine_specs

* Refine select_star

* Fix execute and add more docstrings

* Revert kwargs change from execute

* Remove redundant or

* Align view and table getter schema types

* Fix return type of latest_partition

* Remove some typing from presto

* Improve docstring for __extract_from_token

superset/db_engine_specs/bigquery.py

@@ -14,8 +14,10 @@
 # KIND, either express or implied.  See the License for the
 # specific language governing permissions and limitations
 # under the License.
+from datetime import datetime
 import hashlib
 import re
+from typing import Any, Dict, List, Tuple
 
 import pandas as pd
 from sqlalchemy import literal_column
@@ -56,28 +58,29 @@ class BigQueryEngineSpec(BaseEngineSpec):
     }
 
     @classmethod
-    def convert_dttm(cls, target_type, dttm):
+    def convert_dttm(cls, target_type: str, dttm: datetime) -> str:
         tt = target_type.upper()
         if tt == "DATE":
             return "'{}'".format(dttm.strftime("%Y-%m-%d"))
         return "'{}'".format(dttm.strftime("%Y-%m-%d %H:%M:%S"))
 
     @classmethod
-    def fetch_data(cls, cursor, limit):
+    def fetch_data(cls, cursor, limit: int) -> List[Tuple]:
         data = super(BigQueryEngineSpec, cls).fetch_data(cursor, limit)
         if data and type(data[0]).__name__ == "Row":
             data = [r.values() for r in data]
         return data
 
     @staticmethod
-    def mutate_label(label):
+    def _mutate_label(label: str) -> str:
         """
         BigQuery field_name should start with a letter or underscore and contain only
         alphanumeric characters. Labels that start with a number are prefixed with an
         underscore. Any unsupported characters are replaced with underscores and an
         md5 hash is added to the end of the label to avoid possible collisions.
-        :param str label: the original label which might include unsupported characters
-        :return: String that is supported by the database
+
+        :param label: Expected expression label
+        :return: Conditionally mutated label
         """
         label_hashed = "_" + hashlib.md5(label.encode("utf-8")).hexdigest()
 
@@ -93,15 +96,20 @@ class BigQueryEngineSpec(BaseEngineSpec):
         return label_mutated
 
     @classmethod
-    def truncate_label(cls, label):
+    def _truncate_label(cls, label: str) -> str:
         """BigQuery requires column names start with either a letter or
         underscore. To make sure this is always the case, an underscore is prefixed
-        to the truncated label.
+        to the md5 hash of the original label.
+
+        :param label: expected expression label
+        :return: truncated label
         """
         return "_" + hashlib.md5(label.encode("utf-8")).hexdigest()
 
     @classmethod
-    def extra_table_metadata(cls, database, table_name, schema_name):
+    def extra_table_metadata(
+        cls, database, table_name: str, schema_name: str
+    ) -> Dict[str, Any]:
         indexes = database.get_indexes(table_name, schema_name)
         if not indexes:
             return {}
@@ -136,11 +144,11 @@ class BigQueryEngineSpec(BaseEngineSpec):
         ]
 
     @classmethod
-    def epoch_to_dttm(cls):
+    def epoch_to_dttm(cls) -> str:
         return "TIMESTAMP_SECONDS({col})"
 
     @classmethod
-    def epoch_ms_to_dttm(cls):
+    def epoch_ms_to_dttm(cls) -> str:
         return "TIMESTAMP_MILLIS({col})"
 
     @classmethod