input: 
  def eval_expr(expr: str) -> float:
      """ Evaluate a string containing a numerical expression and return the result.
      >>> eval_expr('2 + 3 * 4')
      14
      >>> eval_expr('(2 + 3) * 4')
      20
      >>> eval_expr('5 / 2')
      2.5
      """
      return eval(expr)
  
output:     return eval(expr)