change an exception's message and re-raise it

[Cameron Simpson]

On 31Dec2009 12:18, Phlip <phlip2005 at gmail.com> wrote:
| Pythonistas:
| 
| I need to do this:
| 
|   try:
|      deep_arcane_layer()
|   except e:
|     e.message = 'the deep arcane layer says: ' + e.message
|     raise e
| 
| The point is I need to augment that layer's exceptions with extra
| information that I know about that layer.

Doesn't the above work (modulo changing the right exception attributes)?

I've got a context manager that does something like this.
It is hampered by the fact that not all exceptions have the same
internal structure, for historic reasons I believe.

So I've got this ugly stuff in the __exit__ method:

  if exc_value is not None:
    if hasattr(exc_value, 'args') and len(exc_value.args) > 0:
      exc_value.args = [pfx + ": " + str(exc_value.args[0])] \
                     + list(exc_value.args[1:])
    else:
      # we can't modify this - at least report the current prefix
      # state
      sys.stderr.write("%s: Pfx.__exit__: exc_value = %s\n" % (pfx,
repr(exc_value),))

You can see I modify the .args value.
The method returns False to let the exception percolate back out
the stack.

The "else" part is to handle exceptions I can't change, so for now I've
just printing the "pfx" context on stderr as the exception bubbles out.
Nasty.

The calling code looks like this:

  with Pfx("some tag"):
    ... suite ...

Cheers,
-- 
Cameron Simpson <cs at zip.com.au> DoD#743
http://www.cskk.ezoshosting.com/cs/

There is this special biologist word we use for 'stable'. It is 'dead'.
        - Jack Cohen