code: def safe_serialize_type(l):
      '''serialize only with letters, numbers and _'''
  
      if isinstance(l, str):
          return l
      elif isinstance(l, list):
          return '%s_%s_' % (l[0], ''.join(map(safe_serialize_type, l[1:])))
      else:
          return str(lf safe_serialize_type(l):
      '''serialize only with letters, numbers and _'''
  
      if isinstance(l, str):
          return l
      elif isinstance(l, list):
          return '%s_%s_' % (l[0], ''.join(map(safe_serialize_type, l[1:])))
      else:
          return str(l)
docs: serialize only with letters, numbers and _