try/except. Our version of the oops function (file oops.py) follows. As for the noncoding questions, changing oops to raise KeyError instead of IndexError means that the exception won't be caught by the 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. Import __builtin__ and pass it as an argument to the dir function to see for yourself.

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!

Exception objects and lists. Here's the way we extended this module for an exception of our own (here a string, at first):

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

To identify the exception with a class, we just changed the first part of the file to this, and saved it as oop_oops.py:

class MyError: pass

def oops(  ):
    raise MyError(  )

...rest unchanged...

Like all class exceptions, the instance comes back as the extra data; our error message now shows both the class, and its instance (<...>).

% python oop_oops.py
caught error: __main__.MyError <__main__.MyError instance at 0x00867550>

Remember, to make this look nicer, you can define a __repr__ or __str__ method in your class to return a custom print string. See Chapter 21 for details.

Error handling. Here's one way to solve this one (file safe2.py). We did our tests in a file, rather than interactively, but the results are about the same.

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 (innermost last):
  File "safe2.py", line 5, in safe
    apply(entry, args)                     # catch everything else
  File "oops.py", line 4, in oops
    raise MyError, 'world'
hello: world
Got hello world