code: def test_dense_matrix_from_nested_dictionary():
      d = {"a": {"b": 10}, "b": {"c": 20}}
      X, rows, columns = dense_matrix_from_nested_dictionary(d)
      eq_(rows, ["a", "b"])
      eq_(columns, ["b", "c"])
      eq_(X[0, 0], 10)
      assert np.isnan(X[0, 1])
      assert np.isnan(X[1, 0])
      eq_(X[1, 1], 20)
  
file_path: fancyimpute/test/test_dictionary_helpers.py
imports: import numpy
  from fancyimpute.dictionary_helpers import dense_matrix_from_pair_dictionary, dense_matrix_from_nested_dictionary, reverse_lookup_from_nested_dict, transpose_nested_dictionary
  from nose.tools import eq_
label_desc: Use image node_id 1 for calling a global function with example usage: test_dense_matrix_from_nested_dictionary() without return types