Leo Breebaart <[EMAIL PROTECTED]> wrote:
> I've recently become rather fond of using Exceptions in Python to
> signal special conditions that aren't errors, but which I feel
> are better communicated up the call stack via the exception
> mechanism than via e.g. return values.
>
> For instance, I'm thinking of methods such as:
>
>
> def run(self):
> """ Feed the input file to the simulator. """
>
> for linenr, line in enumerate(self.infile):
> try:
> current_values = self.parse_line(linenr, line)
> ==> except CommentLineException:
> continue
> results = self.do_one_simulation_step(current_values)
> self.process_simulation_results(results)
I know this isn't the question you asked, but I would have written a little
generator function which hides the comment processing in a lower level:
def nonCommentLines (file):
for lineNumber, line in enumerate (file):
if not line.startswith('#'):
yield lineNumber, line
for lineNumber, line in nonCommentLines (sys.stdin):
current_values = self.parse_line(linenr, line)
results = self.do_one_simulation_step(current_values)
self.process_simulation_results(results)
This assumes you don't mind your line numbers starting from zero, but your
version has the same behavior.
> My question is twofold. First, I know that many programmers are
> violently opposed to using exceptions in this fashion, i.e. for
> anything other than, well, exceptional circumstances. But am I
> correct in thinking that in Python this usage is actually
> considered quite normal, and not frowned upon? Is my snippet
> above indeed sufficiently Pythonic?
I think my code is clearer, but I wouldn't go so far as to say I'm
violently opposed to your code. I save violent opposition for really
important matters like which text editor you use.
I suspect if you showed your code to a C++ guy, he might be violently
opposed. In C++, exceptions are perceived to be more heavyweight than they
are in Python. Some of this is just perception (perhaps colored by
historical issues with older C++ compilers not handling exceptions well),
some of it is real. Just the other day, a colleague of mine was telling me
about a bug in his code. It happened because he had failed to write a copy
constructor for an exception class he wrote. The mind boggles how anybody
can get anything useful done in a language like that. But I digress.
In Python, exceptions seem to be the much more accepted way of doing
things. You often see:
try:
value = dictionary[key]
except KeyError:
whatever
instead of:
if dictionary.has_key (key):
value = dictionary[key]
else:
whatever
While in C++, the "if" version would be far more common.
> Second, purely as a question of aesthetics, I was wondering if
> the folks here might have any opinions about actual naming
> conventions for the exception classes used in this fashion.
If you look at the list of standard exceptions (import exceptions and do a
dir() on it), you'll see things like KeyboardInterrupt, StopIteration, and
SystemExit. If the name fits, raise it.
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