Remove comments from queries in SQL Lab that break Explore view (#4413)

* Remove comments from queries in SQL Lab that break Explore view

This fixes an issue where comments on the last line of the source query comment out the closing parenthesis of the subquery.

* Add test case for SqlaTable with query with comment

This test ensures that comments in the query are removed when calling SqlaTable.get_from_clause().

* Add missing blank line class definition (PEP8)

tests/core_tests.py

@@ -18,6 +18,7 @@ import unittest
 from flask import escape
 import pandas as pd
 import psycopg2
+from six import text_type
 import sqlalchemy as sqla
 
 from superset import appbuilder, dataframe, db, jinja_context, sm, sql_lab, utils
@@ -870,6 +871,13 @@ class CoreTests(SupersetTestCase):
             {'data': pd.Timestamp('2017-11-18 22:06:30.061810+0100', tz=tz)},
         )
 
+    def test_comments_in_sqlatable_query(self):
+        clean_query = "SELECT '/* val 1 */' as c1, '-- val 2' as c2 FROM tbl"
+        commented_query = '/* comment 1 */' + clean_query + '-- comment 2'
+        table = SqlaTable(sql=commented_query)
+        rendered_query = text_type(table.get_from_clause())
+        self.assertEqual(clean_query, rendered_query)
+
 
 if __name__ == '__main__':
     unittest.main()