Core Python Programming (2nd Edition)

10.9. *Creating Exceptions
Although the set of standard exceptions is fairly wide-ranging, it may be advantageous to create your
own exceptions. One situation is where you would like additional information from what a standard or
module-specific exception provides. We will present two examples, both related to IOError.
IOError is a generic exception used for input/output problems, which may arise from invalid file access
or other forms of communication. Suppose we wanted to be more specific in terms of identifying the
source of the problem. For example, for file errors, we want to have a FileError exception that behaves
like IOError, but with a name that has more meaning when performing file operations.
Another exception we will look at is related to network programming with sockets. The exception
generated by the socket module is called socket.error and is not a built-in exception. It is subclassed
from the generic Exception exception. However, the exception arguments from socket.error closely
resemble those of IOError exceptions, so we are going to define a new exception called NetworkError,
which subclasses from IOError but contains at least the information provided by socket.error.
Like classes and object-oriented programming, we have not formally covered network programming at
this stage, but skip ahead to Chapter 16 if you need to.
We now present a module called myexc.py with our newly customized exceptions FileError and
NetworkError. The code is in Example 10.2.
Example 10.2. Creating Exceptions (myexc.py)
This module defines two new exceptions,FileError and NetworkError, as well as
reimplements more diagnostic versions of open() [myopen()] and socket.connect()
[myconnect()]. Also included is a test function [test()] that is run if this module is executed
directly.
1 #!/usr/bin/env python
2
3 import os, socket, errno, types, tempfile
4
5 class NetworkError(IOError):
6 pass
7
8 class FileError(IOError):
9 pass
10
11 def updArgs(args, newarg=None):
12 if isinstance(args, IOError):
13 myargs = []
14 myargs.extend([arg for arg in args])
15 else:
16 myargs = list(args)
17
18 if newarg:
19 myargs.append(newarg)
20