Changes in __getitem__ in Python 2.0?
tim.one at home.com
Wed Feb 28 08:43:26 CET 2001
[Stephen W. Juranich]
> It's been a while since I've been playing with my Python toys (I've been
> banished to C++ land for a while <retch>). I have a class with the
> following definition for __getitem__:
> def __getitem__(self, key):
> # In some cases, I'm getting requests for data that are
> # past the end of the list. Given the sloppy nature of
> # the hand-labelled data, I don't think this is too much
> # of a problem, so I'll try this scheme.
> retval = self.data[key]
> except IndexError:
> retval = self.data[-1]
> return retval
> I'm now encountering a problem with Python 2.0 that I never had
> with earlier versions (1.5 and 1.6 specifically).
Sorry, but this is hard to believe. You Changed Something.
> I used to be able to do something like this:
> for foo in MyClass:
> print foo.member,
I'm assuming you meant to say that you're iterating over an *instance* of the
class that contains the __getitem__ shown above.
> But now when I do this, Python just starts spinning out of control in an
> infinite loop
Under the assumption above, it *should* be an infinite loop. Also in 1.6,
1.5, ..., and 0.9.6. Nothing relevant has ever changed here. The "for"
protocol doesn't stop until __getitem__ raises an IndexError. The only way
your __getitem__() above can raise an IndexError is if self.data is empty:
then the first iteration will try self.data, that will raise IndexError,
but then you'll catch IndexError, then the code will try self.data[-1], and
that will raise an uncaught IndexError (which terminates the loop). So your
loop should terminate if and only if self.data is empty.
> I am assuming that the problem is because of the __getitem__ definition
> above. Is there a way around this, or am I going to have to
> change a bunch of code to make this work on a new Python setup?
Again, it has nothing to do with 2.0, but you're not going to believe that
until you accept my challenge to show us a complete piece of executable code
that demonstrates the problem under 2.0 but not under earlier Pythons <wink>.
If your __getitem__ never raises IndexError, your "for" loop will never end;
that's always been the case.
or-the-problem-isn't-where-you-think-it-is-ly y'rs - tim
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