<<Python基础教程>>学习笔记 | 第08章 | 异常

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什么是异常:Python用异常对象(exception object)来表示异常情况.如果异常信息未被处理或捕捉。

程序就会用回潄来终止执行

>>> 1/0
Traceback (most recent call last):   #Traceback: 一种错误信息
  File "<stdin>", line 1, in ?
ZeroDivisionError: integer division or modulo by zero

Note:ZeroDivisionError:就是一个实例

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按照自己的方式出错

raise语句:

为了引发异常，可以使用一个类或者实例参数调用raise语句。使用类时，程序会自动创建实例，下面是一些简单的例子，使用了内建的Exception异常类

#引发一个没有任何信息的普通异常

>>> raise Exception
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
Exception

>>> raise Exception,"This is a normal Exception"
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
Exception: This is a normal Exception

>>> raise Exception('System device Busy...')
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
Exception: System device Busy...

>>> dir(exceptions)
['ArithmeticError', 'AssertionError', 'AttributeError', 'DeprecationWarning', 'EOFError', 'EnvironmentError', 'Exception', 'FloatingPointError', 'FutureWarning', 'IOError', 'ImportError', 'IndentationError', 'IndexError', 'KeyError', 'KeyboardInterrupt', 'LookupError', 'MemoryError', 'NameError', 'NotImplementedError', 'OSError', 'OverflowError', 'OverflowWarning', 'PendingDeprecationWarning', 'ReferenceError', 'RuntimeError', 'RuntimeWarning', 'StandardError', 'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError', 'SystemExit', 'TabError', 'TypeError', 'UnboundLocalError', 'UnicodeDecodeError', 'UnicodeEncodeError', 'UnicodeError', 'UnicodeTranslateError', 'UserWarning', 'ValueError', 'Warning', 'ZeroDivisionError', '__doc__', '__name__']

1.所有这些异常都可以用在raise语句中.

2. 最重要的一些内建异常类

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自定义异常类:

如果内建的异常类不能满足需要,那么就需要独立于exceptions外的异常类，也就是自定义类.

可以添加异常类，像下面的模式

>>> class customerException(Exception): pass

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捕捉异常

异常最有用的地方是能被捕捉，可以用try/except来实现，比如说很两个数相除

>>> x = input('Enter the first number:')
Enter the first number:10
>>> y = input('Enter the second number:')
Enter the second number:0
>>> print x/y
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ZeroDivisionError: integer division or modulo by zero.

>>> try:
...     x = input('Enter the 1st number:')
...     y = input('Enter the 2nd number:')
        print x/y
... except ZeroDivisionError:
...     print "The second number can't be zero!"
...
Enter the 1st number:10
Enter the 2nd number:0
The second number can't be zero!

也就是说你可以捕捉到在其他人的函数所引发的异常。

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看，没有参数

如果捕捉到异常，又想重新引发它，那么就可以调用不带参数的raise,举个例子吧，看这么做，多有用！考虑一个能屏蔽ZeroDivisionError的计算器类，如果这个错误被激活，计算器就会打印出错误，而不是让异常传播，如果这是在用户交互的过程中使用，这就非常有用，但是，如果在程序内部使用，引发异常会更好些，那么屏蔽机制就可以关掉。下面是这样一个类的代码

class MuffledCalculator:
    muffled = False
    def cal(self,expr):
        try:
            return eval(expr)
        except ZeroDivisionError:
            if self.muffled:
                print 'Division by zero is illegal'
            else:
                raise

输出结果

>>> c = MuffledCalculator()
>>> c.cal('10/2')
5
>>> c.cal('10/0')
Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    c.cal('10/0')
  File "D:\Learn\Python\Exception.py", line 13, in cal
    return eval(expr)
  File "<string>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero
>>> c.muffled = True
>>> c.cal('10/0')
Division by zero is illegal

多个except语句

>>> x = input('Enter the 1st number:')
Enter the 1st number:10
>>> y = input('Enter the 2nd number:')
Enter the 2nd number:'hello,world!'
>>>
>>> print x/y
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: unsupported operand type(s) for /: 'int' and 'str'

try:
    x = input('Enter the 1st number:')
    y = input('Enter the 2nd number:')
    print x/y
except ZeroDivisionError:
    print "The second number can't be zero!"
except TypeError:
    print "That wasn't a number, was it?"

用一个快来捕捉多个异常

try:
    x = input('Enter the 1st number:')
    y = input('Enter the 2nd number:')
    print x/y
#拿所有可能的错误，放在元祖里面，
#注意，一定要用()，否则可能出现不可预知的错误
except (ZeroDivisionError,TypeError,NameError):
    print "Your numbers were bogus..."

捕捉对象

如果希望在except子句中访问异常对象本身，可以使用两个参数。比如，如果想让程序继续运行，但是又因为某种原因想记录下错误（比如只是打印给用户看），这个功能就很有用。下面的例子可以打印异常，但程序会继续运行。

try:
    x = input('Enter the 1st number:')
    y = input('Enter the 2nd number:')
    print x/y
except (ZeroDivisionError,TypeError,NameError), e:
    print e
print 'Hello,World!'

>>> 
Enter the 1st number:10
Enter the 2nd number:0
integer division or modulo by zero
'Hello,World!'
>>> 
Enter the 1st number:10
Enter the 2nd number:hello
name 'hello' is not defined
'Hello,World!'

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真正的全捕捉:

就算程序能处理好几个异常，但有些异常还是会从眼皮底下溜走，还是刚才那个除法的程序，如果什么都不输入呢？ 

Traceback (most recent call last):
  File "<pyshell#0>", line 2, in <module>
    x = input('Enter the 1st number:')
  File "<string>", line 0
    
   ^
SyntaxError: unexpected EOF while parsing

try:
    x = input('Enter the 1st number:')
    y = input('Enter the 2nd number:')
    print x/y
except:
    print "Something Wrong Happen!"

WRAN: 像这样捕捉所有异常是危险的，因为它会隐藏所有程序员未想到并且未做好准备处理的错误。它同样会捕捉用户终止执行CTRL+C企图，以及用sys.exit函数终止程序的企图，等等。这是用except Exception,e会更好些，或者对异常对象e进行一些检查。

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万事大吉:

有些时候一些坏事发生时执行一段代码会很有用，这时候可以像条件，循环一样，加个else语句

>>> try:
...     print "A Simple"
... except:
...     print "What? Something went wrong?"
... else:
...     print "Ah...Runing as planed."
...
A Simple
Ah...Runing as planed.

while True:
    try:
        x = input('Enter 1st number:')
        y = input('Enter 2nd number:')
        value = x/y
        print 'x/y is:', value
    except:
        print 'Invalid input, please try again!'
    else:
        break

>>> 
Enter 1st number:10
Enter 2nd number:0
Invalid input, please try again!
Enter 1st number:10
Enter 2nd number:e
Invalid input, please try again!
Enter 1st number:10
Enter 2nd number:2
x/y is: 5

while True:
    try:
        x = input('Enter 1st number:')
        y = input('Enter 2nd number:')
        value = x/y
        print 'x/y is:', value
    except Exception, e:
        print 'Invalid input', e
        print 'please try again!'
    else:
        break

Enter 1st number:10
Enter 2nd number:0
Invalid input integer division or modulo by zero
please try again!
Enter 1st number:10
Enter 2nd number:hello
Invalid input name 'hello' is not defined
please try again!
Enter 1st number:10
Enter 2nd number:2
x/y is: 5

最后

最后是finally子句,她可以用在可能的异常清理，也可以用在关闭数据库链接，关闭文件句柄等。一般是try/finally联合使用

x = None
try:
    x = 1/0
finally:
    print "Cleaning Up..."
    del x

运行这段代码，在程序崩溃之前，对于变量x的清理就完成了。

输出结果:

>>> 
Cleaning Up...

Traceback (most recent call last):
  File "D:\Learn\Python\Exception.py", line 72, in <module>
    x = 1/0
ZeroDivisionError: integer division or modulo by zero

try:
    1/0
except Exception, e:
    print e
else:
    print "That's well well."
finally:
    print "Clean Up..."

>>> 
integer division or modulo by zero
Clean Up...

异常和函数

异常和函数能很自然的工作。如果异常在函数内引发而不被处理，她就会传播至函数调用的地方。如果在哪里也没有处理异常，她会继续传播，一直到达主程序。如果那里也没有异常处理程序，程序会带着堆栈跟踪终止。

>>> def faulty():
...     raise Exception('Something is wrong.')
...
>>> def ignore_exception():
...     faulty()
...
>>> def handle_exception():
...     try:
...         faulty()
...     except:
...         print 'Exception handled'
...

>>> ignore_exception()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 2, in ignore_exception
  File "<stdin>", line 2, in faulty
Exception: Something is wrong.
>>> handle_exception()
Exception handled

同样的，她也传播到了handle_exception()，但是也个函数被try/except处理了。

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异常之禅

有些时候if/else实现会比try/except实现起来更好，让我们看看下面几个例子:

def describePerson(person):
    print 'Description of',person['name']
    print 'Age:', person['age']
    if 'occupation' in person:
        print 'Occupation:',person['occupation']
>>> d = {'name':'Alice','age':28}
>>> describePerson(d)
Description of Alice
Age: 28
>>> d = {'name':'Alice','age':28,'occupation':'IT'}
>>> describePerson(d)
Description of Alice
Age: 28
Occupation: IT

一次是查看键是否存在，另外一次是获取值

def describePerson(person):
    print 'Description of',person['name']
    print 'Age:', person['age']
    try:
        print 'Occupation:'+ person['occupation']
    except KeyError:
        pass

try:
    obj.write
except AttributeError:
    print 'The object is not writeable'
else:
    print 'The object is writeable.'

本章函数

warning.filterwarnings(action)             用于过滤警告