code: import numpy as np
  
  from pandas import Categorical, Series
  
  
  def test_nunique():
      # basics.rst doc example
      series = Series(np.random.randn(500))
      series[20:500] = np.nan
      series[10:20] = 5000
      result = series.nunique()
      assert result == 11
  
  
  def test_nunique_categorical():
      # GH#18051
      ser = Series(Categorical([]))
      assert ser.nunique() == 0
  
      ser = Series(Categorical([np.nan]))
      assert ser.nunique() == 0
  