try:
<语块句1>
except:
<语块句2>
else:
<语块句3>
finally:
<语句块4>
finally 对应语块句4一定执行，else对应语块句3不发生异常时执行
实例：身体质量指数BMI（Body Mass Index）
heigh，weight = eval(input("请输入身高（米）和体重（公斤 ）[逗号隔开]："))
bmi = weight/pow(heigh,2);
print("BMI数值为:{.2f}".format(bmi));
who nat = "",""
if bmi < 18.5:
who,nat = "偏瘦","偏瘦"
elif 18.5 <= bmi <24:
who,nat = "正常","正常"
elif 24 <= bmi <25:
who,nat = "正常","偏胖"
elif 25 <= bmi <28:
who,nat = "偏胖","偏胖"
elif 28 <= bmi <30:
who,nat = "偏胖","肥胖"