详解在Python中处理异常的教程
什么是异常？
异常是一个事件，其中一个程序，破坏程序的指令的正常流的执行过程中而发生的。一般情况下，当一个Python脚本遇到一些情况不能处理，就抛出一个异常。异常是一个Python对象，它表示一个错误。
当Python脚本抛出一个异常，它必须处理异常，否则将立即终止。
处理异常：
如果有可能会引发异常的一些可疑的代码，就可以通过将可疑的代码在一个try块：保卫你的程序。在try块，包括以下情况except：语句，其次是代码，作为优雅的处理问题，尽可能块。
语法
这里是try....except...else 块的简单语法：
try: You do your operations here; ...................... except ExceptionI: If there is ExceptionI, then execute this block. except ExceptionII: If there is ExceptionII, then execute this block. ...................... else: If there is no exception then execute this block.
这里有一些关于上述语法要点：
- 单个try语句可以有多个不同的语句。当try块中包含可能会引发不同类型的异常语句，这是很有用的。
- 也可以提供一个通用的except子句，它用来处理任何异常。
- except子句后，可以包括其他子句。块没有引发异常：在别的块中的代码，如果在try中的代码执行。
- 在else块是不需要try：块的代码的保护。
例子
这里是简单的例子，这将打开一个文件并写入内容的文件中并移出正常：
#!/usr/bin/python
try:
fh = open("testfile", "w")
fh.write("This is my test file for exception handling!!")
except IOError:
print "Error: can\'t find file or read data"
else:
print "Written content in the file successfully"
fh.close()
这将产生以下结果：
Written content in the file successfully
示例：
这里有一个更简单的例子，它试图打开没有权限并在文件中写入内容，所以它会引发一个异常：
#!/usr/bin/python
try:
fh = open("testfile", "r")
fh.write("This is my test file for exception handling!!")
except IOError:
print "Error: can\'t find file or read data"
else:
print "Written content in the file successfully"
这将产生以下结果：
Error: can't find file or read data
在except子句无异常：
还可以使用不同的定义如下无异常的声明：
try: You do your operations here; ...................... except: If there is any exception, then execute this block. ...................... else: If there is no exception then execute this block.
try-except 语句捕获所有出现的异常。使用这种try-except 声明不被认为是一个良好的编程习惯，但因为它捕获所有异常，但不会使程序员找出可能出现的问题的根本原因。
在except子句的多个异常：
也可以使用相同的除语句来处理多个异常，如下所示：
try: You do your operations here; ...................... except(Exception1[, Exception2[,...ExceptionN]]]): If there is any exception from the given exception list, then execute this block. ...................... else: If there is no exception then execute this block.
try-finally 语句:
可以使用finally:块连同try:块。在try块是否引发异常或没有任何代码 finally块是一个必须执行的块。try-finally语句的语法是这样的：
try: You do your operations here; ...................... Due to any exception, this may be skipped. finally: This would always be executed. ......................
请注意，可以提供except子句或finally子句，但不能同时使用。不能同时使用else子句与finally子句。
例子：
#!/usr/bin/python
try:
fh = open("testfile", "w")
fh.write("This is my test file for exception handling!!")
finally:
print "Error: can\'t find file or read data"
如果没有权限，以写入方式打开文件，那么这将产生以下结果：
Error: can't find file or read data
同样的例子可以写入更简洁，如下所示：
#!/usr/bin/python
try:
fh = open("testfile", "w")
try:
fh.write("This is my test file for exception handling!!")
finally:
print "Going to close the file"
fh.close()
except IOError:
print "Error: can\'t find file or read data"
当一个异常被抛出在try块中，执行立即传递到finally块。finally块中的所有语句都执行，该异常被再次抛出，并在被处理 except 语句如果出现在一个更高的层在try-except语句。
Exception参数：
异常可以有一个参数，参数是一个值，它给出了关于这个问题的其他信息。参数按异常内容改变。可以通过不同的子句提供一个变量，如下所示捕获异常的参数：
try: You do your operations here; ...................... except ExceptionType, Argument: You can print value of Argument here...
如果正在编写代码来处理一个异常，可以有一个变量按照异常的名称在不同的声明。如果捕捉多个异常，可以有一个变量按照异常的元组。
这个变量将接收主要包含异常原因的异常值。该变量可以在一个元组的形式接收一个或多个值。该元组通常包含错误串，错误码和一个错误的位置。
示例：
下面是一个异常的例子：
#!/usr/bin/python
# Define a function here.
def temp_convert(var):
try:
return int(var)
except ValueError, Argument:
print "The argument does not contain numbers\n", Argument
# Call above function here.
temp_convert("xyz");
这将产生以下结果:
The argument does not contain numbers invalid literal for int() with base 10: 'xyz'
抛出异常：
可以通过使用raise语句抛出几个方面的异常。一般raise语句的语法。
语法
raise [Exception [, args [, traceback]]]
这里，Exception是异常的类型（例如，NameError）和参数是用于异常的参数值。该参数是可选的;如果未提供，则异常的参数是None。
最后一个参数traceback，也是可选的（并且在实践中很少使用），并且如果存在的话，那么用于异常回溯对象。
例子：
异常可以是一个字符串，一个类或一个对象。大多数Python核心抛出是类，有参数认为是类的实例的异常。定义新的异常是很容易的，可以参考如下：
def functionName( level ): if level < 1: raise "Invalid level!", level # The code below to this would not be executed # if we raise the exception
注：为了捕获一个异常，“except”语句必须引用抛出类对象或简单的字符串相同的异常。例如，捕捉到上面的异常，必须写except子句，如下所示：
try: Business Logic here... except "Invalid level!": Exception handling here... else: Rest of the code here...
用户定义的异常：
Python中，还可以通过内置的异常标准的派生类来创建自己的异常。
下面是有关RuntimeError一个例子。这里是从RuntimeError子类的类被创建。当需要显示更多的具体信息时，一个异常被捕获，这是很有用的。
在try块中，用户定义的异常引发，并夹在except块。变量e被用来创建类Networkerror的一个实例。
class Networkerror(RuntimeError): def __init__(self, arg): self.args = arg
所以一旦在上面定义的类，可以按如下方法抛出异常：
try:
raise Networkerror("Bad hostname")
except Networkerror,e:
print e.args
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