Missing something on exception handling in Python 3

[MRAB]

On 27/08/2013 02:48, Skip Montanaro wrote:
> I have code in the pylockfile package that looks roughly like this
> (apologies for the indentation - I'm growing to dislike Gmail for
> typing code fragments):
>
> try:
>      write_pid_to_lockfile(somefile)
> except OSError as exc:
>      if conditions_i_can_handle:
>         do_other_stuff...
>      else:
>          raise LockFailed("Failed to create %s" % self.path)
>
> Thinking I would just get the final exception (not the one I caught
> originally), I was surprised to see this output on my screen:
>
>>>> lock.acquire()
> Traceback (most recent call last):
>    File "lockfile/pidlockfile.py", line 80, in acquire
>      write_pid_to_pidfile(self.path)
>    File "lockfile/pidlockfile.py", line 163, in write_pid_to_pidfile
>      pidfile_fd = os.open(pidfile_path, open_flags, open_mode)
> OSError: [Errno 13] Permission denied: '/tmp/skip/lock'
>
> During handling of the above exception, another exception occurred:
>
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
>    File "lockfile/pidlockfile.py", line 94, in acquire
>      raise LockFailed("failed to create %s" % self.path)
> lockfile.LockFailed: failed to create /tmp/skip/lock
>
> When I rung the same code with Python 2.7, I get the exception output
> I anticipate:
>
>>>> lock.acquire()
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
>    File "lockfile/pidlockfile.py", line 94, in acquire
>      raise LockFailed("failed to create %s" % self.path)
> LockFailed: failed to create /tmp/skip/lock
>
> It appears exception handling changed in Python 3.  How do I suppress
> the lower level OSError?
>
Do this:

     raise LockFailed("Failed to create %s" % self.path) from None