语法错误 和 异常
assert（断言）用于判断一个表达式，表达式结果为 false 时 触发异常

try except finally 
raise 

with  自动清理

with open("/tmp/hello.txt","r") as f:
  contents = f.read()
  print(contents)



def errorTest(num):
    try :
        if num >= 10: raise Exception("too big")
    except :
        print("error")
    else :
        print("no error")
    finally :
         print("finnaly run")


errorTest(1)
errorTest(3)
errorTest(20)

except 可同时处理多个异常
except (RuntimeError,TypeError,NameError) :
  pass

except OSError as error :
  print("os error:{0}".format(error))
except ValueError:
  print("value error")
except :
  print("unexpected error")