This question came up again recently over in #27814, in the context of a proposal to add an "unless" parameter to contextlib.suppress(). I declined the RFE mainly on the basis of API complexity, but I also noted you can get something comparable in the current API by using virtual subclassing to say "If a subclass of these, but not of these": http://bugs.python.org/issue27814#msg273434

So the status quo is currently giving us a slightly odd discrepancy between normal except clauses and code that emulates them via issubclass()