input: Write a function to evaluate a given string containing a numerical expression and return the result.
  
  assert eval_expr("1 + 1")==2
  assert eval_expr("2 * 3")==6
  assert eval_expr("5 / 2")==2.5
  assert eval_expr("10 - 4")==6
  assert eval_expr("0")==0
output: def eval_expr(expression):
      return eval(expression)