Kelson Zawack, 25.10.2010 12:33:
The example I have in mind is list like [2,2,2,2,2,2,1,3,3,3,3] where
you want to loop until you see not a 2 and then you want to loop until
you see not a 3. In this situation you cannot use a for loop as
follows:
foo_list_iter = iter([2,2,2,2,2,2,1,3,3,3,3])
for foo_item in foo_list_iter:
if foo_item != 2:
break
because it will eat the 1 and not allow the second loop to find it.
takeWhile and dropWhile have the same problem. It is possible to use
a while loop as follows:
foo_list_item = foo_list_iter.next()
while foo_list_item == 2:
foo_list_item = foo_list_iter.next()
while foo_list_item == 3:
foo_list_item = foo_list_iter.next()
Why not combine the two into this:
foo_list_iter = iter([2,2,2,2,2,2,1,3,3,3,3])
for item in foo_list_iter:
if item != 2:
print item
break
# not sure what to do with the "1" here, hope you do ...
for item in foo_list_iter:
if item != 3:
print item
Also take a look at the itertools module, which provide little helpers like
takewhile(), groupby() and some other nice filters.
As for the feasibly of implementing a has_next function I agree that
you cannot write one method that will provide the proper functionality
in all cases, and thus that you cannot create a has_next for
generators. Iterators however are a different beast, they are
returned by the thing they are iterating over and thus any special
cases can be covered by writing a specific implementation for the
iterable in question.
Well, then just write the specific iterable wrappers that you need for your
specific case. If you do it well, you can make your code a lot easier to
read that way. And that's usually a very good proof that something like
"has_next()" is not needed at all and just complicates things.
This sort of functionality is possible to
implement, because java does it.
Like that was an argument for anything. Java iterators also require you to
implement "remove()". I can't remember ever doing anything else but
throwing an exception in my implementation for that. Bad design isn't a
good reason for a feature to be copied.
Stefan
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