异常处理

　　

错误Error:

• 逻辑错误: 算法错误, - => +
• 笔误: 变量名错误,语法错误
• 函数或类使用错误
• 错误可以避免

异常Exception:

• 本身就是意外情况
• 前提时没有出现上面的错误,即程序写的没有问题,但是某些情况下,会出现一些意外,导致程序无法正常执行下去 
• open函数操作一个文件,文件不存在,或者创建一个文件时,文件已经存在了,或者访问一个网络文件,断网了,这都是异常,是意外情况
• 异常不可能避免

错误和异常:

在高级编程语言中,一般都有错误和异常的概念,异常可以捕获,并加以处理,但是错误是不能被捕获的

异常: 编译器对异常具有跟踪机制

 

 

 

错误:

 

 

 

产生异常:

• raise 语句显式抛出异常
• Python解释器自己检测到异常并引发

def ore():
    print('before')
    def lap():
        print(1/0)
    lap()
    print('after')
ore()

 

 

 

def ore():
    print('before')
    raise Exception('exception!')
    print('after')
ore()

程序会在异常抛出的地方中断执行,如果不捕获异常,就会提前结束程序

 

raise语句:

raise后什么都没有,表示抛出最近一个被激活的异常,如果没有被激活的异常,则抛RuntimeError异常

raise后要求应该是BaseException类或其子类或它们的实例,如果是类,将被无参实例化

try:
    1/0
except BaseException:
    try:
        raise
    except Exception as f:
        print(f)

 

 

BaseException
+-- SystemExit
+-- KeyboardInterrupt
+-- GeneratorExit
+-- Exception
+-- StopIteration
+-- StandardError
| +-- BufferError
| +-- ArithmeticError
| | +-- FloatingPointError
| | +-- OverflowError
| | +-- ZeroDivisionError
| +-- AssertionError
| +-- AttributeError
| +-- EnvironmentError
| | +-- IOError
| | +-- OSError
| | +-- WindowsError (Windows)
| | +-- VMSError (VMS)
| +-- EOFError
| +-- ImportError
| +-- LookupError
| | +-- IndexError
| | +-- KeyError
| +-- MemoryError
| +-- NameError
| | +-- UnboundLocalError
| +-- ReferenceError
| +-- RuntimeError
| | +-- NotImplementedError
| +-- SyntaxError
| | +-- IndentationError
| | +-- TabError
| +-- SystemError
| +-- TypeError
| +-- ValueError
| +-- UnicodeError
| +-- UnicodeDecodeError
| +-- UnicodeEncodeError
| +-- UnicodeTranslateError
+-- Warning
+-- DeprecationWarning
+-- PendingDeprecationWarning
+-- RuntimeWarning
+-- SyntaxWarning
+-- UserWarning
+-- FutureWarning
+-- ImportWarning
+-- UnicodeWarning
+-- BytesWarning
+– ResourceWarning

 

 

异常的捕获:

try:
    code block
except [except type]:
    dealing except

try:
    print('aaaaaaaaaaa')
    print(1/0)
    print('bbbbbbbbbbb')
except:
    print('catch exception')
print('outer')

捕获指定类型的异常

try:
    print('aaaaaaaaaaa')
    print(1/0)
    print('bbbbbbbbbbb')
except ArithmeticError:
    print('catch exception')
print('outer')

 

异常类及继承层次

BaseException
+-- SystemExit
+-- KeyboardInterrupt
+-- GeneratorExit
+-- Exception
+-- StopIteration
+-- StandardError
| +-- BufferError
| +-- ArithmeticError
| | +-- FloatingPointError
| | +-- OverflowError
| | +-- ZeroDivisionError
| +-- AssertionError
| +-- AttributeError
| +-- EnvironmentError
| | +-- IOError
| | +-- OSError
| | +-- WindowsError (Windows)
| | +-- VMSError (VMS)
| +-- EOFError
| +-- ImportError
| +-- LookupError
| | +-- IndexError
| | +-- KeyError
| +-- MemoryError
| +-- NameError
| | +-- UnboundLocalError
| +-- ReferenceError
| +-- RuntimeError
| | +-- NotImplementedError
| +-- SyntaxError
| | +-- IndentationError
| | +-- TabError
| +-- SystemError
| +-- TypeError
| +-- ValueError
| +-- UnicodeError
| +-- UnicodeDecodeError
| +-- UnicodeEncodeError
| +-- UnicodeTranslateError
+-- Warning
+-- DeprecationWarning
+-- PendingDeprecationWarning
+-- RuntimeWarning
+-- SyntaxWarning
+-- UserWarning
+-- FutureWarning
+-- ImportWarning
+-- UnicodeWarning
+-- BytesWarning
+– ResourceWarning

python所有的标准异常类：

异常名称	描述
BaseException	所有异常的基类
SystemExit	解释器请求退出
KeyboardInterrupt	用户中断执行(通常是输入^C)
Exception	常规错误的基类
StopIteration	迭代器没有更多的值
GeneratorExit	生成器(generator)发生异常来通知退出
SystemExit	Python 解释器请求退出
StandardError	所有的内建标准异常的基类
ArithmeticError	所有数值计算错误的基类
FloatingPointError	浮点计算错误
OverflowError	数值运算超出最大限制
ZeroDivisionError	除(或取模)零 (所有数据类型)
AssertionError	断言语句失败
AttributeError	对象没有这个属性
EOFError	没有内建输入,到达EOF 标记
EnvironmentError	操作系统错误的基类
IOError	输入/输出操作失败
OSError	操作系统错误
WindowsError	系统调用失败
ImportError	导入模块/对象失败
KeyboardInterrupt	用户中断执行(通常是输入^C)
LookupError	无效数据查询的基类
IndexError	序列中没有没有此索引(index)
KeyError	映射中没有这个键
MemoryError	内存溢出错误(对于Python 解释器不是致命的)
NameError	未声明/初始化对象 (没有属性)
UnboundLocalError	访问未初始化的本地变量
ReferenceError	弱引用(Weak reference)试图访问已经垃圾回收了的对象
RuntimeError	一般的运行时错误
NotImplementedError	尚未实现的方法
SyntaxError	Python 语法错误
IndentationError	缩进错误
TabError	Tab 和空格混用
SystemError	一般的解释器系统错误
TypeError	对类型无效的操作
ValueError	传入无效的参数
UnicodeError	Unicode 相关的错误
UnicodeDecodeError	Unicode 解码时的错误
UnicodeEncodeError	Unicode 编码时错误
UnicodeTranslateError	Unicode 转换时错误
Warning	警告的基类
DeprecationWarning	关于被弃用的特征的警告
FutureWarning	关于构造将来语义会有改变的警告
OverflowWarning	旧的关于自动提升为长整型(long)的警告
PendingDeprecationWarning	关于特性将会被废弃的警告
RuntimeWarning	可疑的运行时行为(runtime behavior)的警告
SyntaxWarning	可疑的语法的警告
UserWarning	用户代码生成的警告

 

 

BaseException及其子类

所有内建异常类的基类是BaseException

SystemExit

sys.exit()函数引发的异常,异常捕获处理,就直接交给python解释器,解释器退出

import sys
import time
try:
    while 1:
        time.sleep(1)
        print('^'*40)
except KeyboardInterrupt:
    print('bbbbbbbbbbbbbbbbbbb')

 

Exception及子类

Exception是所有内建的,非系统退出的异常的基类,自定义异常应该继承自它

SyntaxError语法错误是不能捕获的

 

ArithmeticError :

算术计算引发的异常,其子类有除零异常等

LookupError:

使用映射的键或序列的索引无效时引发的异常的基类: IndexError       KeyError

class MyException(Exception):
    pass
try:
    raise MyException
except MyException as e:
    print(MyException.mro())

 

 

异常的捕获

except可以捕获多个异常

class MyException(Exception):
    pass
try:
    1/0
    raise MyException
    open('uiop')
except ZeroDivisionError:
    print(1/0)
except MyException as e:
    print(MyException.mro())
except Exception:
    print('Exception')

 

f=None
try:
    f=None
    f=open('uiop')
except OSError as e:
    print(e.args)
finally:
    if f is not None:
        f.close()
    print(f)
print(dir()) # 查看f位于何作用域

class MyException(Exception):
    def __init__(self):
        self.e=a

try:
    raise MyException
except MyException as e:
    print(e,type(e))
except Exception as e:
    print('^'*80)
    print(e,type(e))

 

 raise中构造MyException实例的时候,产生NameError异常,被raise收集

 

 

class MyException(Exception):
    def __init__(self,e):
        self.e=e

try:
    raise MyException
except MyException as e:
    print(e,type(e))
# except Exception as e:
#     print('^'*80)
#     print(e,type(e))

　　

 

 捕获到了TypeError异常 

自定义异常类:

class MyException(Exception):
    def __init__(self,code,msg):
        self.code=code
        self.msg=msg
try:
    raise MyException(22,'uiop')
except MyException as e:
    print(e.code,e.msg)

 

finally子句

try:
    f=None
    f=open('uiop')
except FileNotFoundError as e:
    print('{} {} {}'.format(e.__class__,e.errno,e.strerror))
finally:
    print('cleanup')
    if f is not None:
        f.close()

 

try:
    f=open('uiop')
except:
    pass
finally:
    try:
        f.close()
    except:
        pass
print('outer')

try:
    f=open('uiop')
except Exception as e:
    print('e: {}'.format(e))
finally:
    try:
        f.close()
    except NameError as e:
        print(e)

 

 finally返回值

def ore():
    try:
        return 22
    except:
        pass
    finally:
        print('finally')
        # return 33

print(ore()) # 观察返回值

 

 

def ore():
    try:
        return 22
    except:
        pass
    finally:
        print('finally')
        return 33

print(ore()) # 观察返回值

 

 

 

import time,threading
def f1():
    try:
        1/0
    finally:
        print(11111111111)

def f2():
    time.sleep(4)
    try:
        f1()
    finally:
        print(22222222222222222)
        open('uioop')
    while True:
        time.sleep(1)

t=threading.Thread(target=f2,)
t.start()

while True:
    time.sleep(1)
    print('~~~'*40)
    if t.is_alive():
        print('alive')
    else:
        print('dead')

?

1 2 3 4 5 6 7 8 9 10 11 12 13

try:     try:         ret=1/0     except KeyError as e:         print(e)     else:         print('inner ok')     finally:         print('inner finsh') except:     print('outer catch') finally:     print('outer finsh')

　　

内部捕获不到异常,会向外层传递异常

但是如果内层有finally且其中有return,break,则异常不会继续向外抛出

def f():
    try:
        ret=1/0
    except KeyError as e:
        print(e.__slots__)
    finally:
        print('inner finish')
        return 88888

try:
    print(f())
except:
    print('outer catch')
finally:
    print('outer finish')

 

异常的捕获时机

 　　

def f():
    try:
        ret=1/0
    except KeyError as e:
        print(e.__slots__)
    finally:
        print('inner finish')
        for b in range(5):
            if b>3:
                print(b)
                break
try:
    print(f())
except:
    print('outer catch')
finally:
    print('outer finish')

 

print(type(NotImplemented))
print(type(NotImplementedError))

NotImplemented 是个值,单值,是NotImplementedType的实例
NotImplementedError 是个类型,是异常