On Thu, 14 May 2009 00:39:35 -0400, ma wrote:
> A really great use for try/except/else would be if an object is
> implementing its own __getitem__ method, so you would have something
> like this:
>
> class SomeObj(object):
> def __getitem__(self, key):
> try:
> #sometype of assertion here based on key type
> except AssertionError, e:
> raise TypeError, e #invalid type
Why raise AssertionError only to catch it and raise TypeError? Why not
just raise TypeError in the first place?
If you're thinking of writing this:
assert isinstance(key, whatever)
you should be aware that when Python is run with the -O flag (optimize),
all asserts are disabled, and so your error checking code will not run
and your program will crash and burn in a horrible flaming mess.
[st...@wow-wow ~]$ python -O
Python 2.5 (r25:51908, Nov 6 2007, 16:54:01)
[GCC 4.1.2 20070925 (Red Hat 4.1.2-27)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> assert None is 2
>>>
>>>
The assert statement is not for runtime error checking. assert is for
checking your program logic and invariants. If you've ever written a
comment like:
# When we get here, then x will always be greater than 3.
(or something like that), that's a great candidate for an assertion:
assert x > 3, "x is unexpectedly less than or equal to three"
For error checking, you should do something like this:
if not isinstance(key, whatever):
raise ValueError
rather than:
try:
if not isinstance(key, whatever):
raise AssertionError
except AssertionError:
raise ValueError
--
Steven
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