Path: csiph.com!usenet.pasdenom.info!weretis.net!feeder4.news.weretis.net!rt.uk.eu.org!newsfeed.xs4all.nl!newsfeed3.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
To: python-list@python.org
From: Ben Finney <ben+python@benfinney.id.au>
Subject: Re: Using "with" context handler, and catching specific exception?
Date: Tue, 22 Oct 2013 14:53:58 +1100
References: <a2623010-97d3-4a6d-9373-e9979492b84f@googlegroups.com> <mailman.1331.1382411066.18130.python-list@python.org> <829a6777-3410-49ea-8652-ea1e4ceb9cd2@googlegroups.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.4 (gnu/linux)
Cancel-Lock: sha1:WHXxaZPCsoQS686qpePKF/NA37M=
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.1334.1382414051.18130.python-list@python.org>
Lines: 46
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: csiph.com comp.lang.python:57231

Victor Hooi <victorhooi@gmail.com> writes:

> Aha, good point about IOError encapsulating other things, I'll use
> FileNotFoundError, and also add in some other except blocks for the
> other ones.

Or not; you can catch OSError, which is the parent of FileNotFoundError
<URL:http://docs.python.org/3/library/exceptions.html#exception-hierarchy>,
but don't assume in your code that it means anything more specific.

You should only catch specific exceptions if you're going to do
something specific with them. If all you want to do is log them and move
on, then catch a more general class and ask the exception object to
describe itself (by using it in a string context).


In versions of Python before 3.3, you have to catch EnvironmentError
<URL:http://docs.python.org/3.2/library/exceptions.html#EnvironmentError>
and then distinguish the specific errors by their ‘errno’ attribute
<URL:http://docs.python.org/3.2/library/errno.html>::

    import errno

    try:
        with open('somefile.log', 'wb') as f:
            f.write("hello there")
    except EnvironmentError as exc:
        if exc.errno == errno.ENOENT:
            handle_file_not_found_error()
        elif exc.errno == errno.EACCES:
            handle_permission_denied()
        elif exc.errno == errno.EEXIST:
            handle_file_exists()
        …
        else:
            handle_all_other_environment_errors()

That's much more clumsy, which is why it was improved in the latest
Python. If you can, code for Python 3.3 or higher.

-- 
 \     “Unix is an operating system, OS/2 is half an operating system, |
  `\    Windows is a shell, and DOS is a boot partition virus.” —Peter |
_o__)                                                        H. Coffin |
Ben Finney