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+3")==5
  assert eval_expr("10-5")==5
  assert eval_expr("3*4")==12
  assert eval_expr("15/3")==5.0
  assert eval_expr("0/1")==0.0
output: def eval_expr(expr):
     return eval(expr)