code: def test_fix_dtypes(self, params):
      op = SqlToS3Operator(
          query="query",
          s3_bucket="s3_bucket",
          s3_key="s3_key",
          task_id="task_id",
          sql_conn_id="mysql_conn_id",
      )
      dirty_df = pd.DataFrame(
          {"strings": ["a", "b", None], "ints": [1, 2, None]}
      )
      op._fix_dtypes(df=dirty_df, file_format=params["file_format"])
      assert (
          dirty_df["strings"].values[2] == params["null_string_result"]
      )
      assert dirty_df["ints"].dtype.kind == "i"
  
file_path: airflow/tests/providers/amazon/aws/transfers/test_sql_to_s3.py
imports: from __future__ import annotations
  from tempfile import NamedTemporaryFile
  from unittest import mock
  import pandas
  import pytest
  from airflow.exceptions import AirflowException
  from airflow.models import Connection
  from airflow.providers.amazon.aws.transfers.sql_to_s3 import SqlToS3Operator
label_desc: Use image node_id 4 for calling the TestSqlToS3Operator obj's underlying member method code with example usage: obj.test_fix_dtypes(params) without return types