docs: def absolute_error(y, y_pred):
      """Calculates the sum of the differences
      between target and prediction.
      
      Parameters:
      -----------
      y : vector, shape (n_samples,)
      The target values.
  
      y_pred : vector, shape (n_samples,)
      The predicted values.
      
      Returns:
      --------
      error : float number, sum of the differences
      between target and prediction
      
      """
  
      y, y_pred = convert_assert(y, y_pred)
      return np.sum(y - y_pred)