code: def test_convert_cat_to_string():
      X = pd.DataFrame({"a": [1, 2, 3, "1", 2, 3, "a", 1, 2, 2, 3]})
      X_clean = clean(X)
      assert len(X_clean.a.cat.categories) == 4
  
imports: import os
  import pytest
  import random
  import string
  import warnings
  import numpy
  import pandas
  from sklearn.datasets import load_iris, load_digits
  from sklearn.model_selection import train_test_split
  from sklearn.pipeline import make_pipeline
  from sklearn.linear_model import LogisticRegression
  from dabl.preprocessing import USELESS_TYPES, detect_types, EasyPreprocessor, DirtyFloatCleaner, clean, _FLOAT_REGEX, _float_matching, detect_type_series
  from dabl.utils import data_df_from_bunch
  from dabl.datasets import load_titanic
  from dabl import plot
label_desc: Use image node_id 7 for calling a global function with example usage: test_convert_cat_to_string() without return types