In article <mailman.1415.1305134998.9059.python-l...@python.org>,
Andrew Berg <bahamutzero8...@gmail.com> wrote:
> I'm a bit new to programming outside of shell scripts (and I'm no expert
> there), so I was wondering what is considered the best way to handle
> errors when writing a module. Do I just let exceptions go and raise
> custom exceptions for errors that don't trigger a standard one? Have the
> function/method return nothing or a default value and show an error
> message? I'm sure there's not a clear-cut answer, but I was just
> wondering what most developers would expect a module to do in certain
> situations.
In general, raise an exception when there is no valid value that can be
returned from a function. Sometimes it makes more sense to return 0,
None, an empty list, etc. So:
count_vowels("banana") ==> 3
count_vowels("xyzzy") ==> 0 # counting "y" as not a vowel
count_vowels("") ==> 0
count_vowels(None) ==> raises TypeError
You want to raise specific errors. Let's say you've got a function like
this:
def airspeed(swallow):
speeds = {"european": 42,
"african", 196}
return speeds[swallow]
If somebody passes an invalid string, it will raise KeyError as written.
Better to do something like:
def airspeed(swallow):
speeds = {"european": 42,
"african", 196}
try:
return speeds[swallow]
except KeyError:
raise UnknownBirdError(swallow)
This lets somebody catch UnknownBirdError at some higher level and do
something useful. If you let KeyError escape, that's a lot harder to
deal with because it could come from almost anywhere.
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