code: def test_make_grouping_by_key_default():
      grouping = (0, {"A": 1, "B": 2})
      source = make_key_source(2)
      result = make_grouping_by_key(grouping, source)
      expected = ["a", {"A": "b", "B": None}]
      assert expected == result
  
      # Custom default
      result = make_grouping_by_key(
          grouping, source, default="_missing_"
      )
      expected = ["a", {"A": "b", "B": "_missing_"}]
      assert expected == result
  
imports: from dash.dependencies import Input
  from dash._grouping import flatten_grouping, make_grouping_by_index, grouping_len, map_grouping, make_grouping_by_key, validate_grouping, SchemaTypeValidationError, SchemaLengthValidationError, SchemaKeysValidationError
  from fixtures import scalar_grouping_size, list_grouping_size, dict_grouping_size, mixed_grouping_size
  import string
  import pytest
label_desc: Use image node_id 21 for calling a global function with example usage: test_make_grouping_by_key_default() without return types