docs: def _index(array, item, key=None):
      """
      Array search function.
  
      Written, because ``.index()`` method for array doesn't have `key` parameter
      and raises `ValueError`, if the item is not found.
  
      Args:
          array (list): List of items, which will be searched.
          item (whatever): Item, which will be matched to elements in `array`.
          key (function, default None): Function, which will be used for lookup
                                        into each element in `array`.
  
      Return:
          Index of `item` in `array`, if the `item` is in `array`, else `-1`.
      """
      for i, el in enumerate(array):
          resolved_el = key(el) if key else el
  
          if resolved_el == item:
              return i
  
      return -1