[Python-Dev] Adding content to exception messages

Nicolas Fleury nidoizo at yahoo.com
Tue May 17 07:58:12 CEST 2005

Sorry if this message is not a direct reply to Ka-Ping
Yee message on PEP344, I'm in vacation in China and
there's a thing I must say that could make sense in

I do a lot of exception re-raising at work; I use that
technique to add content to exception messages while
keeping the original stack.  I even created a reraise
function that I use that way:

exeption Exception, exception:
            "Error at line %s in file %s" % (x,y))

(x,y) are details, but you get the idea.

This very useful in many situations.  In the example,
it works when an error is happening in parsing a file
including other files (like xml files with
<xs:include>).  That way you get the exact path of
inclusion leading to the error.  It is also useful
when an error happen in very generic code and when
traceback is not enough to know which element was
causing the error.

What I propose is that all exceptions objects have a
standard way to add additional informations. 
Something like:

except Exception, exception:
    exception.add_info("some info")
    raise exception from (whatever the proposition is)

You might ask, "why not just reraise a new
exception?".  It is more useful to reraise the same
exception type, making it possible to use selective
except clauses and (avoid problems with code using
them like hasattr).  I think what would be simpler is
not affect __str__ representation and prepend to a
list the additional infos inside the exception object,
adding a function to get these infos.  I don't mind
how it would be done in fact, as long as the need is
fulfilled.  I won't be able to read my messages often
for the next 10 days, but I hope others will see the
point I try to bring;)


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