a few extensions for the itertools

Mathias Panzenboeck e0427417 at student.tuwien.ac.at
Mon Nov 20 16:37:45 CET 2006


Steven D'Aprano wrote:
> On Mon, 20 Nov 2006 09:54:41 +0100, Mathias Panzenboeck wrote:
> 
>> Steven D'Aprano wrote:
>>> On Sun, 19 Nov 2006 21:35:24 +0100, Mathias Panzenboeck wrote:
>>>
>>>> I wrote a few functions which IMHO are missing in python(s itertools).
>>>>
>>>> You can download them here:
>>>> http://sourceforge.net/project/showfiles.php?group_id=165721&package_id=212104
>>>>
>>>> A short description to all the functions:
>>>>
>>>> icmp(iterable1, iterable2) -> integer
>>>> 	Return negative if iterable1 < iterable2,
>>>> 	zero if iterable1 == iterable1,
>>>> 	positive if iterable1 > iterable1.
>>>
>>> What does it mean for an iterable to be less than another iterable? That
>>> it has fewer items? How do these two iterables compare?
>>>
>>> iter([1, 2, None, "foo", 3+2j])
>>>
>>> def ones():
>>>     while 1:
>>>         yield 1
>>>
>>> Which is smaller?
>>>
>>>
>> it's like cmp on lists, but on iterables.
>>
>> [1,2,3] < [1,2,4]
>> [1,2,3] < [1,2,3,0]
> 
> 
> But that meaningless, as far as I can see. Lists and iterators aren't the
> same thing. A list is a collection; an iterator is not, but it can be
> accumulated into a collection.
> 
> If equality is meaningful for an object, you should be able to test for
> equality without changing the object. But that isn't true for iterators.
> 
> Worse, because comparing an iterator consumes items, you can easily get
> crazy results like the following:
> 
> 
>>>> L = [1, 2, 3, 4, 5]
>>>> def it():
> ...     yield 5; yield 0; yield 0; yield 0; yield 0
> ... 
>>>> it = it()
> 
> Now, we can compare the first item of L with the first item of it:
> 
>>>> L[0] < it.next()  # if True, L < it
> True
> 
> So L must be less than it, right? Let's print both objects out in full to
> check:
> 
>>>> print L
> [1, 2, 3, 4, 5]
>>>> print list(it) # if L < it, it must be > L
> [0, 0, 0, 0]
> 
> Oops.
> 
> I think the GENERAL concept of comparison between iterators is
> meaningless. A lazy comparison between the items of an iterator and some
> other iterable may be a useful thing to do, but as a general concept,
> saying that an iterator compares bigger or smaller or equal to something
> else doesn't make sense, since the mere fact that you make that comparison
> will change the iterator.
> 
> 

I see. Yes, in general you are right, but I implemented it to use it for that (and similar things):

icmp(open("foo.txt"),open("bar.txt"))

Thats cool, I think. :)


	panzi



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