mertz at gnosis.cx

[Scott David Daniels]

I was pointed to appendix A by a comp.lang.py post.
In the "except statements" section (which might better be called
the "try" statement and include a try: ... finally: ...), you say:

   The 'except' statement can optionally bind a name to an exception
   argument:

       >>> try:
       ...     raise "ThisError", "some message"
       ... except "ThisError", x:    # Bind 'x' to exception argument
       ...     print x
       ...
       some message

String exceptions should not be encouraged (nor do I think they work).
Better code would be:

       >>> class MyError(Exception): pass
       >>> try:
       ...     raise MyError, "some message"
       ... except MyError, x:    # Bind 'x' to exception instance
       ...     print x
       ...
       some message

or, if you don't want two statements:

       >>> try:
       ...     raise ValueError, "some message"
       ... except ValueError, x:    # Bind 'x' to exception instance
       ...     print x
       ...
       some message


The x, by the way, is bound to an instance of the class of the
exception, and it has a field "args" which will reflect the
arguments with which the exception was created.  So in these
cases, x.args is ('some message',) and if the code were:

       >>> try:
       ...     raise ValueError("some message", 42)
       ... except ValueError, x:    # Bind 'x' to exception instance
       ...     print x
       ...
       ('some message', 42)

and x.args would be: ("some message", 42)


--Scott David Daniels
Scott.Daniels at Acm.Org