Re: iterator clone
On 16 июл, 11:32, Marc 'BlackJack' Rintsch <[EMAIL PROTECTED]> wrote: > On Tue, 15 Jul 2008 19:54:30 -0700, Yosifov Pavel wrote: > > Kay, can you show example of such generator? ReIter, for example, work > > with usual generators. > > > But for "big" iterator, I think is no any good solutions. IMHO we can > > discern 2 types of iterators: re-startable (based on internal Python > > objects) and not re-startable (with an external state, side- > > effects)... > > Has nothing to do with internal vs. external. > Examples: ``itertools.count(1)``, ``itertools.cycle(iterable)``, or > > def fib(): > a, b = 0, 1 > while True: > yield a > a, b = b, a + b > > Ciao, > Marc 'BlackJack' Rintsch Yes. So, I'm disconcerted: what Python "means" about iterators. Why iterator's are not clonable in default?.. ``itertools.count(it) `` "suppose" ``it`` will be restarted after count? So ``count(it)`` "suppose" ``it`` is restarable and therefore clonable (why not?!). Generator is only function, so can be restarted/cloned. Generator keeps "position" and can't be iterated again. But this position can be reseted (main rule: if generator function has not side-effects/ external state, see below)! Iterator, I think, is more general method (for naturally serial access). For example, you can read values from some device (RS323 or other) and represent it in program like iterator. In this case, ``__iter__()`` make some preparation, ``next()`` read next value from RS232. In this case, iterator can't be restarted and really cloned. It has external state (state of device) and can't to return it at start. But when series of values are generated (are born) in the program: no problem to have the feature of clone/restart iterator. And is very interesting to research Icon iterators. Is possible to create something in Python such theirs, for non-deterministic solving purpose... :-) --pavel -- http://mail.python.org/mailman/listinfo/python-list
Re: iterator clone
On Tue, 15 Jul 2008 19:54:30 -0700, Yosifov Pavel wrote: > Kay, can you show example of such generator? ReIter, for example, work > with usual generators. > > But for "big" iterator, I think is no any good solutions. IMHO we can > discern 2 types of iterators: re-startable (based on internal Python > objects) and not re-startable (with an external state, side- > effects)... Has nothing to do with internal vs. external. Examples: ``itertools.count(1)``, ``itertools.cycle(iterable)``, or def fib(): a, b = 0, 1 while True: yield a a, b = b, a + b Ciao, Marc 'BlackJack' Rintsch -- http://mail.python.org/mailman/listinfo/python-list
Re: iterator clone
Kay, can you show example of such generator? ReIter, for example, work with usual generators. But for "big" iterator, I think is no any good solutions. IMHO we can discern 2 types of iterators: re-startable (based on internal Python objects) and not re-startable (with an external state, side- effects)... Best regards, Pavel -- http://mail.python.org/mailman/listinfo/python-list
Re: iterator clone
On 15 Jul., 08:16, Yosifov Pavel <[EMAIL PROTECTED]> wrote: > cloning of iterators in this manner is bad, more good is to use one, > single list(my_iter) instead of > (seehttp://aquagnu.blogspot.com/2008/07/self-repair-iterator-in-python.html). This won't work for "big" iterators as mentioned by Peter Otten. With this recipe you can't even clone generator objects ( which are iterators ) that produce Fibonaccis in a lazy manner. Regards, Kay -- http://mail.python.org/mailman/listinfo/python-list
Re: iterator clone
On 14 июл, 23:36, "[EMAIL PROTECTED]" <[EMAIL PROTECTED]> wrote: > On 13 juil, 12:05, Yosifov Pavel <[EMAIL PROTECTED]> wrote: > (snip) > > > defcloneiter( it ): > > """return (clonable,clone)""" > > return tee(it) > > This might as well be written as > > cloneiter = tee > > Or yet better, just remove the above code and s/cloneiter/tee/g in the > remaining... Yes, sure. It was only for illustration. BUT: Marc Rintsch is right: cloning of iterators in this manner is bad, more good is to use one, single list(my_iter) instead of (see http://aquagnu.blogspot.com/2008/07/self-repair-iterator-in-python.html). Thanks to all! --pavel -- http://mail.python.org/mailman/listinfo/python-list
Re: iterator clone
On 13 Jul., 08:53, Yosifov Pavel <[EMAIL PROTECTED]> wrote: > Whats is the way to clone "independent" iterator? I can't use tee(), > because I don't know how many "independent" iterators I need. copy and > deepcopy doesn't work... > > --pavel You can try generator_tools http://pypi.python.org/pypi/generator_tools/0.3.3 -- http://mail.python.org/mailman/listinfo/python-list
Re: iterator clone
On 13 juil, 12:05, Yosifov Pavel <[EMAIL PROTECTED]> wrote: (snip) > def cloneiter( it ): > """return (clonable,clone)""" > return tee(it) This might as well be written as cloneiter = tee Or yet better, just remove the above code and s/cloneiter/tee/g in the remaining... -- http://mail.python.org/mailman/listinfo/python-list
Re: iterator clone
> `tee()` doesn't copy the iterator or its internal state but just caches > it's results, so you can iterate over them again. That makes only sense > if you expect to use the two iterators in a way they don't get much out of > sync. If your usage pattern is "consume iterator 1 fully, and then > re-iterate with iterator 2" `tee()` has no advantage over building a list > of all results of the original iterator and iterate over that twice. > `tee()` would be building this list anyway. It's interesting and a concrete answer. Thanks a lot. > Because it's often not possible without generating a list with all > results, and the advantage of a low memory footprint is lost. > > Ciao, > Marc 'BlackJack' Rintsch Seems like "monada". But I think is possible to determine when there is a bounded external state (side-effects) or not, may be is needed some new class-protocol for it... or something else. Or another way: iterators may be re-iterable always, but if programmer need to point to the extra- (external) state, he has to raise some a special exception in __iter)) method... OK, it's only fantasies about language design :-) --pavel -- http://mail.python.org/mailman/listinfo/python-list
Re: iterator clone
On Sun, 13 Jul 2008 18:51:19 -0700, Yosifov Pavel wrote:
>> Well, I think Python's iterators, especially the generators, are beautiful.
>> More importantly, I think there is no general way to make iterators
>> copyable, regardless of the programming language. The problem is that most
>> of the useful ones depend on external state.
>
> Hmm, but tee() de facto do it (clone iterator) and ignore side-effects
> of iterator ("external" state). And tee() create independent
> **internal** state of iterator (current position).
`tee()` doesn't copy the iterator or its internal state but just caches
it's results, so you can iterate over them again. That makes only sense
if you expect to use the two iterators in a way they don't get much out of
sync. If your usage pattern is "consume iterator 1 fully, and then
re-iterate with iterator 2" `tee()` has no advantage over building a list
of all results of the original iterator and iterate over that twice.
`tee()` would be building this list anyway.
> But **external** state - is headache of programmer. So,
> iterator/generator have to be method for copy itself (the tee()
> implementation) or be "re- startable". Why not?
Because it's often not possible without generating a list with all
results, and the advantage of a low memory footprint is lost.
Ciao,
Marc 'BlackJack' Rintsch
--
http://mail.python.org/mailman/listinfo/python-list
Re: iterator clone
> Well, I think Python's iterators, especially the generators, are beautiful.
> More importantly, I think there is no general way to make iterators
> copyable, regardless of the programming language. The problem is that most
> of the useful ones depend on external state.
>
> Peter
Hmm, but tee() de facto do it (clone iterator) and ignore side-effects
of iterator ("external" state). And tee() create independent
**internal** state of iterator (current position). But **external**
state - is headache of programmer. So, iterator/generator have to be
method for copy itself (the tee() implementation) or be "re-
startable". Why not?
Concrete problem was to generate iterators (iterator of slices). It
was solved with ReIter.
--Best regards,
--pavel
--
http://mail.python.org/mailman/listinfo/python-list
Re: iterator clone
Yosifov Pavel wrote: > On 13 июл, 14:12, Peter Otten <[EMAIL PROTECTED]> wrote: >> Yosifov Pavel wrote: >> > Whats is the way to clone "independent" iterator? I can't use tee(), >> > because I don't know how many "independent" iterators I need. copy and >> > deepcopy doesn't work... >> >> There is no general way. For "short" sequences you can store the items in >> a list which is also the worst-case behaviour of tee(). >> >> What are you trying to do? >> >> Peter > > I try to generate iterators (iterator of iterators). Peter, you are > right! Thank you. For example, it's possible to use something like > this: > > def cloneiter( it ): > """return (clonable,clone)""" > return tee(it) [snip] That is too abstract, sorry. What concrete problem are you trying to solve with your cloned iterators? There might be a way to rearrange your setup in a way that doesn't need them. > But I think (I'm sure!) it's deficiency of Python iterators! They are > not very good... Well, I think Python's iterators, especially the generators, are beautiful. More importantly, I think there is no general way to make iterators copyable, regardless of the programming language. The problem is that most of the useful ones depend on external state. Peter -- http://mail.python.org/mailman/listinfo/python-list
Re: iterator clone
On 13 июл, 14:12, Peter Otten <[EMAIL PROTECTED]> wrote: > Yosifov Pavel wrote: > > Whats is the way to clone "independent" iterator? I can't use tee(), > > because I don't know how many "independent" iterators I need. copy and > > deepcopy doesn't work... > > There is no general way. For "short" sequences you can store the items in a > list which is also the worst-case behaviour of tee(). > > What are you trying to do? > > Peter I try to generate iterators (iterator of iterators). Peter, you are right! Thank you. For example, it's possible to use something like this: def cloneiter( it ): """return (clonable,clone)""" return tee(it) and usage: clonable,seq1 = cloneiter(seq) ...iter over seq1... then clone again: clonable,seq2 = cloneiter(clonable) ...iter over seq2... Or in class: class ReIter: def __init__( self, it ): self._it = it def __iter__( self ): self._it,ret = tee(self._it) return ret and usage: ri = ReIter(seq) ...iter over ri... ...again iter over ri... ...and again... But I think (I'm sure!) it's deficiency of Python iterators! They are not very good... --Pavel -- http://mail.python.org/mailman/listinfo/python-list
Re: iterator clone
Yosifov Pavel wrote: > Whats is the way to clone "independent" iterator? I can't use tee(), > because I don't know how many "independent" iterators I need. copy and > deepcopy doesn't work... There is no general way. For "short" sequences you can store the items in a list which is also the worst-case behaviour of tee(). What are you trying to do? Peter -- http://mail.python.org/mailman/listinfo/python-list
iterator clone
Whats is the way to clone "independent" iterator? I can't use tee(), because I don't know how many "independent" iterators I need. copy and deepcopy doesn't work... --pavel -- http://mail.python.org/mailman/listinfo/python-list
