[issue15806] Add context manager for the try: ... except: pass pattern
Changes by Alexander Belopolsky alexander.belopol...@gmail.com: -- superseder: - Rename contextlib.ignore to contextlib.suppress ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Changes by Giampaolo Rodola' g.rod...@gmail.com: -- nosy: +giampaolo.rodola ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Yury V. Zaytsev added the comment: Hi Raymond, This is a brilliant idea, but before it hits the streets, couldn't you possibly consider extending it with a kwarg to control the depth of the exception stack? The use case I have for that are snippets like this: with ignored(ValueError, TypeError), ignored(ValueError, TypeError), ignored(ValueError, TypeError): a() b() c() Or else I could write this as with ignored(ValueError, TypeError): a() with ignored(ValueError, TypeError): b() with ignored(ValueError, TypeError): c() ... but either way it looks bad. This looks a bit better to me: with ignored(ValueError, TypeError, depth=3): a() b() c() If you deem this to be unacceptably unpythonic, then please ignore my suggestion. -- nosy: +zaytsev ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Yury V. Zaytsev added the comment: Actually, please disregard my idea. It's way to dangerous, especially in the case of multiple exceptions to ignore :-( -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Changes by Barry A. Warsaw ba...@python.org: -- nosy: +barry ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Nick Coghlan added the comment: FTR, Raymond and I discussed this on IRC and I gave it a +1 before he committed it. The advantage the callable form has over a class method is that it readily scales to ignoring multiple exception types in a single with statement. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Roundup Robot added the comment: New changeset 406b47c64480 by Raymond Hettinger in branch 'default': Issue #15806: Add contextlib.ignored(). http://hg.python.org/cpython/rev/406b47c64480 -- nosy: +python-dev ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Changes by Raymond Hettinger raymond.hettin...@gmail.com: -- resolution: - fixed status: open - closed ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Raymond Hettinger added the comment: Yes, something along those lines would be *much* better: class Ignore: ''' Context manager to ignore particular exceptions''' def __init__(self, *ignored_exceptions): self.ignored_exceptions = ignored_exceptions def __enter__(self): return self def __exit__(self, exctype, excinst, exctb): return exctype and issubclass(exctype, self.ignored_exceptions) -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Nick Coghlan added the comment: I'd just write it with @contextmanager. Making it easier to cleanly factor out exception handling is one of the main reasons that exists. @contextmanager def ignored(*exceptions): Context manager to ignore particular exceptions try: yield except exceptions: pass While the class based version would likely be fractionally faster, the generator based version is more obviously correct. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Changes by Chris Jerdonek chris.jerdo...@gmail.com: -- nosy: +cjerdonek ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Ezio Melotti added the comment: I think I'm -1 on this. This saves two lines, but makes the code less explicit, it can't be used for try/except/else or try/except/except, it requires an extra import, the implementation is simple enough that it doesn't necessary need to be in the stdlib, it might encourage bad style, and it's slower. Some of these downsides are indeed somewhat weak, but the upside of saving two lines is quite weak too IMHO. -- nosy: +ezio.melotti ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Martin v. Löwis added the comment: I think the zipfile example is really a bad example. IMO, it would best be written as try: return bool(EndRecData(fp)) except IOError: return False i.e. there shouldn't be a pass statement at all in this code, and the if can be dropped whether you use try-except or with. -- nosy: +loewis ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Changes by Chris Rebert pyb...@rebertia.com: -- nosy: +cvrebert ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Nick Coghlan added the comment: (Note: I'm not yet convinced this is a good idea. I'm definitely considering it, though) As with many context managers, a key benefit here is in the priming effect for readers. In this code: try: # Whatever except (A, B, C): pass the reader doesn't know that (A, B, C) exceptions will be ignored until the end. The with statement form makes it clear before you start reading the code that certain exceptions won't propagate: with ignored(A, B, C): # Whatever I'm not worried that it makes things less explicit - it's pretty obvious what a context manager called ignored that accepts an arbitrary number of exceptions is going to do. One other thing it does is interact well with ExitStack - you can stick this in the stack of exit callbacks to suppress exceptions that you don't want to propagate. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Ezio Melotti added the comment: As with many context managers, a key benefit here is in the priming effect for readers. The focus is mostly on what it's being executed rather than what it's being ignored though. Do this operation and ignore these exceptions if they occur vs. Ignore these exceptions if they occur while doing this operation. I'm not worried that it makes things less explicit - it's pretty obvious what a context manager called ignored that accepts an arbitrary number of exceptions is going to do. It's still understandable, but while I'm familiar with the semantics of try/except, I wouldn't be sure if e.g. this just ignored those specific exceptions or even their subclasses without checking the doc/code. One other thing it does is interact well with ExitStack - you can stick this in the stack of exit callbacks to suppress exceptions that you don't want to propagate. This seems a good use case. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Antoine Pitrou added the comment: If this is desirable then I think it would be better as a classmethod of Exception: with KeyError.trap(): do_something() -- nosy: +pitrou ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Eric V. Smith added the comment: While the classmethod version has some appeal, it doesn't extend well to handling multiple exception types. I'm -0 on this, in any event. I think the original code is more clear. Why force people to learn (or recognize) a second idiom for something so simple? -- nosy: +eric.smith ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Changes by Jesús Cea Avión j...@jcea.es: -- nosy: +jcea ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
New submission from Raymond Hettinger: It is a somewhat common pattern to write: try: do_something() except SomeException: pass To search examples in the standard library (or any other code base) use: $ egrep -C2 except( [A-Za-z]+)?: *py | grep -C2 pass In the Python2.7 Lib directory alone, we find 213 examples. I suggest a context manager be added that can ignore specifie exceptions. Here's a possible implementation: class Ignore: ''' Context manager to ignore particular exceptions''' def __init__(self, *ignored_exceptions): self.ignored_exceptions = ignored_exceptions def __enter__(self): return self def __exit__(self, exctype, excinst, exctb): return exctype in self.ignored_exceptions The usage would be something like this: with Ignore(IndexError, KeyError): print(s[t]) Here's a real-world example taken from zipfile.py: def _check_zipfile(fp): try: if _EndRecData(fp): return True # file has correct magic number except IOError: pass return False With Ignore() context manager, the code cleans-up nicely: def _check_zipfile(fp): with Ignore(IOError): return bool(EndRecData(fp)) # file has correct magic number return False I think this would make a nice addition to contextlib. -- assignee: ncoghlan messages: 169337 nosy: ncoghlan, rhettinger priority: low severity: normal status: open title: Add context manager for the try: ... except: pass pattern type: enhancement versions: Python 3.4 ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Changes by Alex Gaynor alex.gay...@gmail.com: -- nosy: +alex ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Raymond Hettinger added the comment: Hmm, the __exit__ method was doing exact matches by exception type, so KeyError wouldn't match LookupError or Exception. There are probably a number of ways to fix this, but it may be easiest to use the builtin exception catching mechanisms: class Ignore: ''' Context manager to ignore particular exceptions''' def __init__(self, *ignored_exceptions): self.ignored_exceptions = ignored_exceptions def __enter__(self): return self def __exit__(self, exctype, excinst, exctb): if exctype is not None: try: raise except self.ignored_exceptions: return True -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15806] Add context manager for the try: ... except: pass pattern
Alex Gaynor added the comment: Why not just: issubclass(exctype, self.exception_types)? -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue15806 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
