code: def test_density(self):
      df = pd.DataFrame(
          {
              "A": SparseArray([1, 0, 2, 1], fill_value=0),
              "B": SparseArray([0, 1, 1, 1], fill_value=0),
          }
      )
      res = df.sparse.density
      expected = 0.75
      assert res == expected
  
imports: import string
  import numpy
  import pytest
  import pandas
  from pandas import SparseDtype
  import pandas._testing
  from pandas.core.arrays.sparse import SparseArray
label_desc: Use image node_id 9 for calling the TestFrameAccessor obj's underlying member method code with example usage: obj.test_density() without return types