[Python-Dev] On suppress()'s trail blazing (was Re: cpython: Rename contextlib.ignored() to contextlib.ignore())

[Terry Reedy]

On 10/17/2013 7:35 PM, Nick Coghlan wrote:
>
> On 18 Oct 2013 06:59, "Xavier Morel" <catch-all at masklinn.net
> <mailto:catch-all at masklinn.net>> wrote:
>  >
>  > On 2013-10-17, at 22:11 , Ethan Furman wrote:
>  >
>  > > On 10/17/2013 01:03 PM, Terry Reedy wrote:
>  > >>
>  > >> class suppress:
>  > >>   def __init__(self, *exceptions):
>  > >>     self.exceptions = exceptions
>  > >>   def __exit__(self, etype, eval, etrace):
>  > >>     return etype in self.exceptions
>  > >
>  > > This fails when etype is a subclass of the exceptions, as mentioned
> in the original issue.

There are two other failures; see below. I know better than to post code 
without testing or saying 'untested'; I just wish I would always remember...

>  > That's fixed by using `issubclass` and does not infirm Terry's point
> does it?
>
> Yeah, it looks like it's worth switching to the class based
> implementation in this case. I guess I'm too accustomed to that being
> the more complex alternative, as I hadn't even tried it :)

With two more changes (add __enter__, account for etype is None), the 
suppress tests pass.

import unittest

class suppress:
     def __init__(self, *exceptions):
         self.exceptions = exceptions
     def __enter__(self): pass
     def __exit__(self, etype, val, tb):
         return etype is None or issubclass(etype, self.exceptions)

class Testsuppress(unittest.TestCase):

     def test_no_exception(self):
         with suppress(ValueError):
             self.assertEqual(pow(2, 5), 32)

     def test_exact_exception(self):
         with suppress(TypeError):
             len(5)

     def test_multiple_exception_args(self):
         with suppress(ZeroDivisionError, TypeError):
             len(5)

     def test_exception_hierarchy(self):
         with suppress(LookupError):
             'Hello'[50]

unittest.main()

 >>> Ran 4 tests...OK

-- 
Terry Jan Reedy