what a cheap rule

Fuzzyman fuzzyman at gmail.com
Fri Nov 26 15:16:01 CET 2010


On Nov 26, 1:10 am, Steven D'Aprano <steve
+comp.lang.pyt... at pearwood.info> wrote:
> On Thu, 25 Nov 2010 08:15:21 -0800, Yingjie Lan wrote:
> > Intuition #1: as if you raise an exception type, and then match that
> > type.
> > It seems that no instances
> > are involved here (Intuitively).
>
> Your intuition is not my intuition, nor does it match what Python
> actually does. You can only go so far on *guessing* what a programming
> statement does, sometimes you need to actually read the Fine Manual.
>
> > See an example code here:
>
> > try: raise KeyError
> > except KeyError: pass
>
> As the documentation states, this creates an instance of KeyError.
>
>     raise evaluates the first expression as the exception object.
>     It must be either a subclass or an instance of BaseException.
>     If it is a class, the exception instance will be obtained when
>     needed by instantiating the class with no arguments.
>
> http://docs.python.org/py3k/reference/simple_stmts.html#the-raise-sta...
>
> So there is no semantic difference between "raise KeyError" and
> "raise KeyError()". Why should there be? What practical difference would
> you expect?
>

There *can* be a difference though.

>>> raise UnicodeDecodeError
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: function takes exactly 5 arguments (0 given)

:-)

>
> You seem to have misunderstood both forms of the raise statement. Should
> we make exceptions illegal because you can't correctly guess what they do?

Sometimes people not being able to understand them is a good reason
for making things illegal (or rather for not making them legal in the
first place). I don't think it applies to this particular case though.

All the best,

Michael Foord

--
http://www.voidspace.org.uk/



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