That's great to know! Thanks for that explanation, I am refactoring something and I was going to make ample use of assertion as I thought it was the same as C's assertion without the NDEBUG flag.
On Thu, May 14, 2009 at 1:03 AM, Steven D'Aprano <ste...@remove.this.cybersource.com.au> wrote: > 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 > -- > http://mail.python.org/mailman/listinfo/python-list > -- http://mail.python.org/mailman/listinfo/python-list
