[Python-3000] Exception tracebacks and PEP 3101 (original) (raw)
Patrick Maupin pmaupin at gmail.com
Tue Mar 6 04:28:04 CET 2007
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I'm a newbie when it comes to deep Python internals, so maybe I'm missing something obvious. I'm trying to figure out the right answer for exceptions/tracebacks in PEP 3101 (advanced string formatting).
Because the PEP 3101 string formatting is so much more powerful than the existing %s, it seems that users will be much more tempted to write large strings with complicated expressions inside them. When an exception is thrown and the user needs to debug such a string, it would be very useful to be able to show the user line number/character information for the failing location inside the format string itself. (The utility of this cannot be overstated -- I believe it will make or break the out-of-the-box experience for this new feature for many users.)
But because Python is so dynamic, if an exception is thrown during an attribute or index operation during a call to the format() function, the exception might be because of something several execution frames deep into the attribute operation, and it might be useful to show the traceback stack to the user.
However, it seems that the Python traceback scheme is intimately tied to the concept of Python execution frames, which are assumed to have associated text information from a file. I'm not sure that I can legitimately add to a preexisting traceback from a C function in a non-fragile fashion. Even if I could, I don't really have a "file" to show the user (unless I dynamically create one).
So it seems the only answer might have to have options like "let the user see the existing traceback" or "get rid of the traceback and raise an exception which shows exactly where in the format string the error occured", or "print the preexisting traceback to stderr, then toast it and raise a new exception showing the string location in the exception text."
I guess there are two parts to my question:
What is the best thing to do right now for PEP3101?
If the "right now" answer isn't very good, is this really a more general problem that needs work? As more "little languages" do things like manipulate the AST, it might become very useful to have the ability to place almost arbitrary objects on the traceback stack.
Thanks, Pat
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