On Oct 18, 2:53 pm, Eloff <dan.el...@gmail.com> wrote:
> On Oct 16, 7:38 pm, Carl Banks <pavlovevide...@gmail.com> wrote:
>
> > You would burden everyone who writes a custom iterator to provide a
> > __getitem__ method just because you're too lazy to type out the word
> > islice?
>
> No, of course not. That would be stupid. Custom iterators are
> iterators, so they would also get the default __getitem__ which would
> work just perfectly for them.
Ok, here's a simple (Python 2.x) custom iterator.
class Alphabet(object):
def __init__(self):
self.c = 65
def __iter__(self):
return self
def next(self):
if self.c > 90:
raise StopIteration
letter = chr(self.c)
self.c += 1
return letter
Let's see what happens *currently* in Python when you try to apply
slice indexing.
for x in AB()[4:9]: # loop through letters 4 through 8
print x
This, as expected, raises a type error:
TypeError: 'AB' object is unsubscriptable
Now, let's try to modify Python to fix this. All we have to do is to
define __getitem__ for the default iterator, so let's define it in the
common iterator type, which is....
Hm, wait a second, problem here. The simple iterator I defined above
isn't an instance of any common iterator type. In fact there is no
common iterator type.
You see, iterator is a *protocol*. An object is not an iterator by
virtue of being an instance of an iterator type, it's an iterator
because it supports a protocol. In fact that protocol is extremely
simple: all an iterator has to do is define __iter__() to return self,
define next() to return next item, and raise StopIteration when it's
done.
So you see, to support slice syntax one would indeed burden all custom
iterators to implement __getitem__ method, which would arguably
increase the implementor's burden by 30%.
> If they needed to override it, they
> could. Remember, my proposal was to add a default __getitem__ for
> iterators that is really just islice.
There is no default __getitem__ for iterators so this proposal won't
work.
Carl Banks
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