Question

I find myself often wanting to structure my exception classes like this:

# legends.py
class Error(Exception): pass

class Rick(object):
    class Error(Error): pass
    class GaveYouUp(Error): pass
    class LetYouDown(Error): pass

class Michael(object):
    class Error(Error): pass
    class BlamedItOnTheSunshine(Error): pass
    class BlamedItOnTheMoonlight(Error): pass

I have only seen this pattern used in Django (DoesNotExist) and it makes so much sense. Is there anything I’m missing, why most people seem to favor top-level Exceptions?

edit
I would use these classes for versatile granularity, e.g:

import legends

try:
    do_stuff()
except legends.Michael.Error:
    blame_it_on_the_boogie()
except legends.Rick.GaveYouUp:
    let_you_down()
except legends.Error:
    pass
except Exception as e:
    raise Hell()

Answer 1

This is the exact pattern used by Django for certain ORM-related exceptions.

The advantage is that you can have an except clause which checks against a type accessed through an instance:

rick = Rick()

try:
   rick.roll()
except rick.GaveYouUp:
   never()
except rick.LetYouDown:
   never_ever()

This doesn’t look that useful here, but if rick were a function parameter, then it would potentially be rather useful.

This is also extremely useful in writing generic code which raises the exceptions:

GoddamStar(object):
   def sing(self,tune):
       raise self.Error()

class Rick(GoddamStar):
    class Error(Error): pass
    class GaveYouUp(Error): pass
    class LetYouDown(Error): pass

class Michael(GoddamStar):
    class Error(Error): pass
    class BlamedItOnTheSunshine(Error): pass
    class BlamedItOnTheMoonlight(Error): pass

rick = Rick()

try:
   rick.sing()
except Rick.GaveYouUp:
   never()
except Michael.Error:
   never_ever()

Django’s exceptions generally all derive from global base classes, so that you can also have a catch-all clause which still switches on a type of exception, in case your rick is of an unknown (or otherwise unprovided for) class.

The reason why this isn’t much more common is that (a) it doesn’t work in early-bound languages, which attract most of the book writers (b) it’s moderately rare that this is useful to the user, and so application writers likely figure they aren’t going to need it.