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("2 + 2")==4
  assert eval_expr("5 * 3")==15
  assert eval_expr("10 / 2")==5
  assert eval_expr("7 - 3")==4
  assert eval_expr("10 % 3")==1
output: def eval_expr(expr):
     return eval(expr)