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"