code: def test_flatten_dict(self):
      """Test dictionary flattening."""
      d = {"a": 1, "b": {"c": 1, "d": {"e": 1, "f": {"g": [1, 2]}}}}
      expected = {"a": 1, "b_c": 1, "b_d_e": 1, "b_d_f_g": [1, 2]}
      assert wutils.flatten_dict(d) == expected
  
imports: import unittest
  import satpy.writers.utils
label_desc: Use image node_id 1 for calling the WriterUtilsTest obj's underlying member method code with example usage: obj.test_flatten_dict() without return types