add comment + a unit test for None values

superset/db_engine_specs/base.py

@@ -743,7 +743,10 @@ def fetch_data(
  return cursor.fetchmany(limit)
  data = cursor.fetchall()
  description = cursor.description or []
- column_type_mutators = {
+ # Create a mapping between column name and a mutator function to normalize
+ # values with. The first two items in the description row are
+ # the column name and type.
+ column_mutators = {
  row[0]: func
  for row in description
  if (
@@ -752,11 +755,11 @@ def fetch_data(
  )
  )
  }
- if column_type_mutators:
+ if column_mutators:
  indexes = {row[0]: idx for idx, row in enumerate(description)}
  for row_idx, row in enumerate(data):
  new_row = list(row)
- for col, func in column_type_mutators.items():
+ for col, func in column_mutators.items():
  col_idx = indexes[col]
  new_row[col_idx] = func(row[col_idx])
  data[row_idx] = tuple(new_row)

tests/unit_tests/db_engine_specs/test_mysql.py

@@ -236,6 +236,11 @@ def test_get_schema_from_engine_params() -> None:
  [("dec", "decimal(12,6)"), ("str", "varchar(3)")],
  [(Decimal("1.23456"), "abc")],
  ),
+ (
+ [[None, "abc"]],
+ [("dec", "decimal(12,6)"), ("str", "varchar(3)")],
+ [(None, "abc")],
+ ),
  (
  [["1.23456", "abc"]],
  [("dec", "varchar(255)"), ("str", "varchar(3)")],

tests/unit_tests/db_engine_specs/test_trino.py

@@ -384,6 +384,11 @@ def test_handle_cursor_early_cancel(
  [("dec", "decimal(12,6)"), ("str", "varchar(3)")],
  [(Decimal("1.23456"), "abc")],
  ),
+ (
+ [[None, "abc"]],
+ [("dec", "decimal(12,6)"), ("str", "varchar(3)")],
+ [(None, "abc")],
+ ),
  (
  [["1.23456", "abc"]],
  [("dec", "varchar(255)"), ("str", "varchar(3)")],