replacing built-in exception types

[Nishkar Grover]

I'm trying to replace a built-in exception type and here's a simplified
example of what I was hoping to do...

>>>
import exceptions, __builtin__

zeroDivisionError = exceptions.ZeroDivisionError

class Foo(zeroDivisionError):

.... bar = 'bar'
....

>>>
exceptions.ZeroDivisionError = Foo
ZeroDivisionError = Foo
__builtin__.ZeroDivisionError = Foo

try:

.... raise ZeroDivisionError
.... except ZeroDivisionError, e:
.... print e.bar
....
bar

>>>
try:

.... 1/0
.... except ZeroDivisionError, e:
.... print e.bar
....
Traceback (most recent call last):
File "<stdin>", line 2, in ?
ZeroDivisionError: integer division or modulo by zero

>>>

Notice that I get my customized exception type when I explicitly raise
ZeroDivisionError but not when that is implicitly raised by 1/0. It
seems like I have to replace that exception type at some lower level,
but I'm not sure how/where. Does anyone know of a way to do this?

- Nishkar

[Stargaming]

On Tue, 11 Dec 2007 12:51:52 -0800, Nishkar Grover wrote:

I'm trying to replace a built-in exception type and here's a simplified
example of what I was hoping to do...

>>import exceptions, __builtin__
>>>
>>zeroDivisionError = exceptions.ZeroDivisionError

I don't know why you're fiddling that much with the names,
`ZeroDivisionError` is a builtin name. If you do this to avoid
overwriting the base classes, consider the following example::

>>class A(object): pass

...

>>class B(A): pass

...

>>A_backup = A
A_backup is A

True

>>class A(object): pass

...

>>A_backup is A

False

>>B.__base__ is A

False

>>B.__base__ is A_backup

True

The names really just point to an object. Once the resolution from a name
to a real object is done, you can reuse the name.

>>class Foo(zeroDivisionError):

... bar = 'bar'
...

>>>
>>exceptions.ZeroDivisionError = Foo
>>ZeroDivisionError = Foo
>>__builtin__.ZeroDivisionError = Foo
>>>
>>try:

... raise ZeroDivisionError
... except ZeroDivisionError, e:
... print e.bar
...
bar

>>>
>>try:

... 1/0
... except ZeroDivisionError, e:
... print e.bar
...
Traceback (most recent call last):
File "<stdin>", line 2, in ?
ZeroDivisionError: integer division or modulo by zero

>>>
>>>

The object that ZeroDivisionError points to in your code is *not* the
raised exception.

Notice that I get my customized exception type when I explicitly raise
ZeroDivisionError but not when that is implicitly raised by 1/0. It
seems like I have to replace that exception type at some lower level,
but I'm not sure how/where. Does anyone know of a way to do this?

The obvious question is: What are you trying to do?

Remember that you can always catch an exception in an `except` clause and
perform whatever action you want, eg. raise *another* exception.

If you just want to change error representations (I doubt that you really
need it, tho), you might be interested in overwriting `sys.excepthook
<http://docs.python.org/lib/module-sys.html#l2h-5125>`_.

HTH,