Python Exception 2018-08-01

1. Routines: using except and adding exception types

# -*- coding: UTF-8 -*-                                          try:    fh = open("a.txt",‘w‘)    fh.write("This is a file for Exception test!")except IOError:    print "Error, file not found or access failure!"else:    print "Successful!"

Output:

[email protected]:~/code$ python test.py Successful![email protected]:~/code$ cat a.txt This is a file for Exception [email protected]

• You can also see the write() non-newline character by opening the A.txt
  1.2. In order to test, we first a.txt to write permission to remove, and then re-execute the above code, you can find the exception caused by the inability to write

chmod -w a.txt[email protected]:~/code$ python test.py Error, file not found or access failure!

1. Use exceptions without any exception type

try:    正常的操作   ......................except:    发生异常，执行这块代码   ......................else:    如果没有异常执行这块代码

1. Using exceptions with multiple exception types

try:    正常的操作   ......................except(Exception1[, Exception2[,...ExceptionN]]]):   发生以上多个异常中的一个，执行这块代码   ......................else:    如果没有异常执行这块代码

1. try...finally statement: The final code will be executed regardless of whether an exception occurs

# -*- coding: UTF-8 -*-                                          try:    fh = open("a.txt",‘w‘)    fh.write("This is a file for Exception test!")finally:    print "Error: File not found or access error"

Output:

[email protected]:~/code$ python test.py Error: File not found or access errorTraceback (most recent call last):  File "test.py", line 3, in <module>    fh = open("a.txt",‘w‘)IOError: [Errno 13] Permission denied: ‘a.txt‘[email protected]:~/code$ chmod +w a.txt [email protected]:~/code$ python test.py Error: File not found or access error

1. Exception parameters: An exception can be taken with parameters, which can be used as an exception parameter for output

try:    正常的操作   ......................except ExceptionType, Argument:    你可以在这输出 Argument 的值...

Instance:

# -*- coding: UTF-8 -*-                                          def temp_convert(var):    try:        return int(var)    except ValueError,Arg:        print "参数不包含数字:",Argprint temp_convert(‘xyz‘)

Output:

[email protected]:~/code$ python test.py 参数不包含数字: invalid literal for int() with base 10: ‘xyz‘None

1. To trigger an exception:

   raise [Exception [, Args [, Traceback]]
   The Exception statement is either of the exception type (for example, nameerror) parameter standard exception, and args is a self-supplied exception parameter.

# -*- coding: UTF-8 -*-def mye(level):    if level < 1:        raise Exception,"Invalid Error!" #触发异常后，后面的代码>将不会被执行try:    mye(0)except Exception,err:    print 1,errelse:    print 2       

Output:

[email protected]:~/code$ python test.py 1 Invalid Error!

1. User-defined exceptions
   By creating a new Exception class, programs can name their own exceptions. Exceptions should be typical of inheriting from the exception class, either directly or indirectly.

• The following are examples of RuntimeError-related instances in which a class is created and the base class is RuntimeError, which is used to output more information when the exception is triggered;
• In the TRY statement block, after the user-defined exception executes the EXCEPT block statement, the variable e is used to create an instance of the Networkerror class.

# -*- coding: UTF-8 -*-class NetworkError(RuntimeError):    def __init__(self, arg):        self.args = argtry:    raise NetworkError("Bad hostName")except NetworkError,e:    print e.args      

Output:

[email protected]:~/code$ python test.py (‘B‘, ‘a‘, ‘d‘, ‘ ‘, ‘h‘, ‘o‘, ‘s‘, ‘t‘, ‘N‘, ‘a‘, ‘m‘, ‘e‘)

Question: Why output a single letter? I'll answer later.

Python Exception 2018-08-01