class_docstr: Test remove digits.
class_docstr_tok: ["Test","remove","digits","."]
code: def test_stopwords_are_set(self):
      self.assertEqual(type(stopwords.DEFAULT), set)
      self.assertEqual(type(stopwords.NLTK_EN), set)
      self.assertEqual(type(stopwords.SPACY_EN), set)
  
imports: import string
  import pytest
  import pandas
  import numpy
  import doctest
  from texthero import preprocessing, stopwords
  from .None import PandasTestCase
  import warnings
label_desc: Use image node_id 16 for calling the TestPreprocessing obj's underlying member method code with example usage: obj.test_stopwords_are_set() without return types