input: Write a function to evaluate a string containing a numerical expression and return the result.
  def eval_expr(expr):
      return eval(expr)
  assert eval_expr("1 + 1")==2
  assert eval_expr("2 * 3")==6
  assert eval_expr("10 / 2")==5
  assert eval_expr("7 - 3")==4
  assert eval_expr("5 ** 2")==25
output: def eval_expr(expr):
      return eval(expr)