Six, Python IO and 5 anomalies, exception handling, and more abnormal capture, exception handling nested custom exception is thrown

5, exception handling, exception capture multiple, nested exception handling, an exception is thrown Custom

1) Exception Handling

try:
    f = open('test.txt', 'r', True, 'GBK')
    print(f.read())
except OSError as e:
    print(e)
    print(e.args)          # 异常参数
    print(e.errno)         # 异常编号
    print(e.strerror)      # 异常描述信息
# 无论正常还是异常，finally 块总会执行，因此通常用于释放资源
finally:
    if 'f' in globals():   # 当 f 变量存在时，关闭 f 文件流
        f.close()     

[Errno 2] No such file or directory: 'test.txt'
(2, 'No such file or directory')
2
No such file or directory

Analysis :

• If you need access exception information, use as abnormal specify a variable name, it can be omitted as clause.

2) Multi abnormal capture

Description : except block may capture a plurality of types of abnormality, a plurality of exception class enclosed in parentheses, separated by commas intermediate (construct multiple tuples exception class)

try:
    a = int(input('请输入第一个整数：'))
    b = int(input('请输入第二个整数：'))
    print(a/b)
except (ValueError, ArithmeticError) as e:
    print(e, type(e))

请输入第一个整数：a
invalid literal for int() with base 10: 'a' <class 'ValueError'>

请输入第一个整数：10
请输入第二个整数：0
division by zero <class 'ZeroDivisionError'>

Analysis : zero exception is the arithmetic exception classes

3) Exception Handling Nested

Description : Contains full exception handling flow in the try block, except the block or blocks finally exception handling nested condition is known.

f = None
try:
    f = open('data.txt', 'r', True)
    print(f.read())
except:
    print('捕捉到异常')
finally:
    if f:       # f 不为 None 时关闭文件
        try:    # 在 finally 块中嵌套了异常处理
            f.close()    
            print('关闭文件')
        except:
            print('关闭文件时有异常')

第一行
第二行
第三行

关闭文件

4) Custom throw an exception: raise

raise statement three common methods :

1. raise: The raise alone triggered the current caught exception, the default trigger abnormal RuntimeError

   # 要求 age 必须在10到30岁之间
   class User:
       def __init__(self,age=25):
           if age>30 or age<10:
               raise            # 默认引发 RuntimeError 异常
           self.__age = age
           
       def setage(self, age):
           if age>30 or age<10:
               raise            # 默认引发 RuntimeError 异常
           self.__age = age
       def getage(self):
           return self.__age
       age = property(fget=getage, fset=setage)
   

   user = User(40)
   

   RuntimeError: No active exception to reraise
   
   

   user = User()
   user.age = 40
   

   RuntimeError: No active exception to reraise
   

2. raise exception classes: with the raise an exception class, default instance of the specified initiator exception class

   class User:
       def __init__(self,age=25):
           if age>30 or age<10:
               raise ValueError    # 引发指定异常类的默认实例
           self.__age = age
           
       def setage(self, age):
           if age>30 or age<10:
               raise ValueError    # 引发指定异常类的默认实例
           self.__age = age
       def getage(self):
           return self.__age
       age = property(fget=getage, fset=setage)
       
   user = User(40)
   

   ValueError: 
   

3. raise exception objects: caused by a specified exception object

   class User:
       def __init__(self,age=25):
           if age>30 or age<10:
               raise ValueError(age, '年龄必须在10~30之间') # 引发指定的异常对象
           self.__age = age
           
       def setage(self, age):
           if age>30 or age<10:
               raise ValueError(age, '年龄必须在10~30之间') # 引发指定的异常对象
           self.__age = age
       def getage(self):
           return self.__age
       age = property(fget=getage, fset=setage)
       
   user = User(40)
   

   ValueError: (40, '年龄必须在10~30之间')
   
   

Analysis : Combining capture abnormal use

class User:
    def __init__(self,age=25):
        if age>30 or age<10:
            raise ValueError(age, '年龄必须在10~30之间') # 引发指定的异常对象
        self.__age = age
        
    def setage(self, age):
        if age>30 or age<10:
            raise ValueError(age, '年龄必须在10~30之间') # 引发指定的异常对象
        self.__age = age
    def getage(self):
        return self.__age
    age = property(fget=getage, fset=setage)
    
user = User(18)
print(user.age)
print('-'*30)

try:
    user.age = 40
except ValueError as e:
    print(e.args)    # args 即创建异常对象时传入的参数

18
------------------------------
(40, '年龄必须在10~30之间')

5) else block

Description : no program exception occurs, the else block

try:
    a = int(input('请输入第一个整数：'))
    b = int(input('请输入第二个整数：'))
    print(a/b)
except (ValueError, ArithmeticError) as e:
    print(e, type(e))
else:
    print('程序一切正常')

请输入第一个整数：10
请输入第二个整数：2
5.0
程序一切正常