Learning Python

Chapter 7

1. try/except. Our version of the oops function follows. As for the noncoding questions, changing oops to raise KeyError instead of IndexError means that the exception won’t be caught by our try handler (it “percolates” to the top level and triggers Python’s default error message). The names KeyError and IndexError come from the outermost built-in names scope. If you don’t believe us, import _ _builtin__ and pass it as an argument to the dir function to see for yourself.

   % cat oops.py
   
   def oops():
       raise IndexError
   
   def doomed():
       try:
           oops()
       except IndexError:
           print 'caught an index error!'
       else:
           print 'no error caught...'
   
   if __name__ == '__main__': doomed()
   
   % python oops.py
   caught an index error!

2. Exception lists. Here’s the way we extended this module for an exception of our own:

   % cat oops.py
   MyError = 'hello'
   
   def oops():
       raise MyError, 'world'
   
   def doomed():
       try:
           oops()
       except IndexError:
           print 'caught an index error!'
       except MyError, data:
           print 'caught error:', MyError, data
       else:
           print 'no error caught...'
   
   if __name__ == '__main__':
       doomed()
   
   % python oops.py
   caught error: hello world

3. Error handling. Finally, here’s one way to solve this one; we decided to do our tests in a file, rather than interactively, but the results are about the same.

   % cat safe2.py
   import sys, traceback
   
   def safe(entry, *args):
       try:
           apply(entry, args)                 # catch everything else
       except:
           traceback.print_exc()
           print 'Got', sys.exc_type, sys.exc_value
   
   import oops
   safe(oops.oops)
   
   % python safe2.py Traceback ...