Python Exception

1 处理exception：

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. 



关键点：

• A single try statement can have multiple except statements. This is useful when the try block contains statements that may throw different types of exceptions.

• You can also provide a generic except clause, which handles any exception.

• After the except clause(s), you can include an else-clause. The code in the else-block executes if the code in the try: block does not raise an exception.

• The else-block is a good place for code that does not need the try: block's protection.

2 except后面没有exception

try:


   You do your operations here;

   ......................

except:


   If there is any exception, then execute this block.

   ......................

else:




不推荐，没有考虑root cause



3 except后面跟着多个exception



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. 



4 try --final

You can use a finally: block along with a try: block. The finally block is a place to put any code that must execute, whether the try-block raised an exception or not. The syntax of the try-finally statement is this:

try: You do your operations here; ...................... Due to any exception, this may be skipped. finally: This would always be executed. ......................

Note that you can provide except clause(s), or a finally clause, but not both. You can not use else clause as well along with a finally clause.

5 exception带参数：

An exception can have an argument , which is a value that gives additional information about the problem. The contents of the argument vary by exception. You capture an exception's argument by supplying a variable in the except clause as follows:

try: You do your operations here; ...................... except ExceptionType, Argument : You can print value of Argument here...

If you are writing the code to handle a single exception, you can have a variable follow the name of the exception in the except statement. If you are trapping multiple exceptions, you can have a variable follow the tuple of the exception.

This variable will receive the value of the exception mostly containing the cause of the exception. The variable can receive a single value or multiple values in the form of a tuple. This tuple usually contains the error string, the error number, and an error location.

Example:

Following is an example for a single exception:

#!/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");

This would produce following result:

The argument does not contain numbers invalid literal for int() with base 10: 'xyz'

 

6 raising an exception

 

raise [Exception [, args [, traceback]]]

Here Exception is the type of exception (for example, NameError) and argument is a value for the exception argument. The argument is optional; if not supplied, the exception argument is None.

The final argument, traceback, is also optional (and rarely used in practice), and, if present, is the traceback object used for the exception

Example:

An exception can be a string, a class, or an object. Most of the exceptions that the Python core raises are classes, with an argument that is an instance of the class. Defining new exceptions is quite easy and can be done as follows:

def functionName( level ): if level < 1: raise "Invalid level!", level # The code below to this would not be executed # if we raise the exception

Note: In order to catch an exception, an "except" clause must refer to the same exception thrown either class object or simple string. For example to capture above exception we must write our except clause as follows:

try:

   Business Logic here...

except "Invalid level!":

   Exception handling here...

else:

   Rest of the code here...