Subclassing exceptions

[Sue Giller]

I am trying to write an exception class that will pass on info from a 
previous exception (if any) along with the currently raised exception. 
 This is in Windows (win32all 1.40)  I am using sys.exc_info() to get 
the current state of the exception.  According to Beazley, pg 118, 
exc_info() should return None if  no exception is currently being 
handled.  See the end of the message for the subclass.

If I catch an exception, and then raise my own Ex exception, things 
seem ok:  (The following prints "Ex: unsupported operand types for 
+; Raising an exception" which is what I want)

try: a = 1 + 'b'
except: raise Ex, "Raising an exception

However, if I just raise my exception, I don't get None back from 
exc_info, but rather a win32ui error: There is no active view".  The 
following prints "Ex: There is no active view.;Raising cain".  I would 
expect to get None since no exception has been raised before my 
exception is raised.

raise Ex, "Raising cain"

A trace on that shows
Traceback (most recent call last):
  File "C:\Python21\Pythonwin\pywin\framework\intpyapp.py", line 
75, in OnCommand
    v = self.GetActiveView() # Raise an exception if none - good - 
then we want default handling
win32ui: There is no active view.

Here is the exception class (test case):  It wants to append any input 
args to any existing text from an exception.

import exceptions
class Ex(exceptions.Exception):    
    def __init__(self, args=None):
        import sys                
        info = sys.exc_info()
        text = ""
        if not info is None:			# always get this!
            text = "%s" % str(info[1])
        if args != None:
            text = text + "\n\t" + args
        self.args = text

Is there some way to write a subclassed exception that allows me to 
pass on the text from the last raised exception (if any) plus my 
subclassed text?

This does not happen from the interactive prompt, but will happen 
from a script.  You can have a simple script in PythonWin, with just 
the lines:

import sys
print sys.exc_info()

and get the same results

('win32ui', 'There is no active view.', <traceback object at 
0133FF70>)