code: def local_pdb_codes(data_dir=None):
      
      if not data_dir:
          data_dir = global_settings["structural_database"]["path"]
      p = Path(data_dir)
      pdb_parent_dirs = [x for x in p.iterdir() if x.is_dir() and len(x.parts[-1]) == 2]
      pdb_folders = [x for test in pdb_parent_dirs for x in test.iterdir() if x.is_dir()]
      pdb_code_list = [x.parts[-1] for x in pdb_folders if len(x.parts[-1]) == 4]
      return pdb_code_list
docs: Get list of PDB codes stored in a folder (FileSystem folder hierarchy expected within data_dir).
  
      If no folder is specified, use the database_dir defined in settings.json.
  
      Parameters
      ----------
      data_dir: str
          Filepath to a folder containing the PDB folder hierarchy (eg data_dir/eb/2ebo)
  
      Returns
      -------
      pdb_code_list : list(str)
          PDB codes present in data_dir.