Python 2.7 allows indexing an Exception object to access its args list.

>>> e=Exception('foo','bar','baz')
>>> assert e[0] is e.args[0]

This doesn't work in 3.5:

>>> e=Exception('foo','bar','baz')
>>> e.args[0]
'foo'
>>> e[0]
TypeError: 'Exception' object is not subscriptable

This conversion (e[x] -> e.args[x]) seems like it ought to be fairly straightforward to support in 2to3.