exception handling in complex Python programs

eliben eliben at gmail.com
Tue Aug 19 19:19:22 CEST 2008


Python provides a quite good and feature-complete exception handling
mechanism for its programmers. This is good. But exceptions, like any
complex construct, are difficult to use correctly, especially as
programs get large.

Most of the issues of exceptions are not specific to Python, but I
sometimes feel that Python makes them more acute because of the free-n-
easy manner in which it employs exceptions for its own uses and allows
users to do the same.

Now, what do I mean more specifically... When a program starts growing
large, I find myself a bit scared of all the exceptions that might be
thrown: Python's exceptions as a result of runtime-detection of errors
(Python's dynamic typing also comes into play here), exceptions from
libraries used by the code, and exceptions from my lower-level
classes.
Python doesn't allow to specify which exceptions are thrown (C++'s
feature adding 'throw' after a function/method declaration specifying
the exceptions that can be thrown), and this leaves me at loss - what
should be caught and where ? Which errors should be left to
propagate ?

I've tried looking around the Python blogosphere, but there doesn't
seem to be much concern with this topic.

Apologies for the not-too-coherent post, but I suspect you feel the
pain too and can understand my meaning.

Eli

P.S. There's a common case where a method is passed a filename, to do
something with a file (say, read data). Should the method catch the
errors possibly thrown by open(), or leave it to the caller ?

P.P.S. There's a great post on conditions (Common Lisp's exceptions)
here:
http://dlweinreb.wordpress.com/2008/03/24/what-conditions-exceptions-are-really-about/
Not really CL specific, and can apply to Python's exceptions.



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