[Python-Dev] Re: Reiterability
aleaxit at yahoo.com
Sun Oct 19 06:05:56 EDT 2003
On Sunday 19 October 2003 00:05, Guido van Rossum wrote:
> > class ReiterableIterator(object):
> > def __init__(self, thecallable, *itsargs, **itskwds):
> Why put support for a callable with arbitrary arguments in the
> ReiterableIterator class? Why not say it's called without args, and
> if the user has a need to use something with args, they can use one of
> the many approaches to currying?
The typical and most frequent case would be that generating a
new iterator requires calling iter(asequence) -- i.e., the typical case
does require arguments. So, just like e.g. for threading.Thread,
atexit.register, and other callables that take a callable argument, it
makes more sense to NOT require the user to invent a currying
approach (note btw that iter does NOT support the iter.__get__ trick,
of course, as it's a builtin function and not a Python function). It
would be different if Python supported a curry built-in, but it doesn't.
> > typical toy example use:
> Are there any non-toy examples?
I have not met any, yet -- whence my interest in hearing about use cases
from anybody who might have.
> I'm asking because I can't remember ever having had this need myself.
Right, me neither.
> A better name would be clone(); copy() would work too, as long as it's
> clear that it copies the iterator, not the underlying sequence or
> series. (Subtle difference!)
> Reiteration is a special case of cloning: simply stash away a clone
> before you begin.
Good name, and good point.
> > Roughly the usual "protocol" approach -- functions use an object's
> > ability IF that object exposes methods providing that ability, and
> > otherwise fake it on their own.
> In this case I'm not sure if it is desirable to do this automatically.
Ah, yes, the automatism might be a performance trap -- good point.
> If I request a clone of an iterator for a data stream coming from a
> pipe or socket, it would have to start buffering everything. Sure, I
> can come up with a buffering class that throws away buffered data that
> none of the existing clones can reach, but I very much doubt if it's
> worth it; a customized buffering scheme for the application at hand
> would likely be more efficient than a generic solution.
Then clone(it) should raise an exception if it does NOT expose a
method supplying "easy cloning" (or more simply it.clone() could
do it, e.g. an AttributeError:-) alerting the user of the need to use
such a "buffering class" wrapper:
try: clo = it.clone()
except AttributeError: clo = BufferingWrapper(it)
But if no existing iterator supplies the .clone -- even when it would
be very easy for it to do so -- this would bufferingwrap everything.
> > > I'm not sure what you are suggesting here. Are you proposing that
> > > *some* iterators (those which can be snapshotted cheaply) sprout a
> > > new snapshot() method?
> > If snapshottability (eek!) is important enough, yes, though
> > __snapshot__ might perhaps be more traditional (but for iterators we do
> > have the precedent of method next without __underscores__).
> (Which I've admitted before was a mistake.)
Ah, I didn't recall that admission, sorry. OK, underscores everywhere then.
> A problem I have with making iterator cloning a standard option is
> that this would pretty much require that all iterators for which
> cloning can be implemented should implement clone(). That in turn
> means that iterator implementors have to work harder (sometimes
> cloning can be done cheaply, but it might require a different
> refactoring of the iterator implementation).
Making iterator authors aware of their clients' possible need to clone
doesn't sound bad to me. There's no _compulsion_ to provide the
functionality, but some "social pressure" to do it if a refactoring can
afford it, well, why not?
> Another issue is that it would make generators second-class citizens,
> since they cannot be cloned. (It would seem to be possible to copy a
> stack frame, but then the question begs whether to use shallow or deep
> copying -- if a local variable in a generator references a list,
> should the list be copied or not? And if it should be copied, should
> it be a deep or shallow copy? There's no good answer without knowing
> the intention of the programmer.)
Hmmm, there's worse -- if a generator uses an iterator the latter should
be cloned, not copied, to produce the generator-clone effect, e.g.
for x in it: yield x*2
If it is a list I don't think this is a problem -- already now the user
cannot change it for the lifetime of iterators produced by by2(it)
without wierd effects, e.g. "for x in by2(L): L.append(x)" gives an
But if it is an iterator it should be cloned at the time an iterator
produced by by2(it) is cloned. Eeep. No, you're right, in the general
case I cannot see how to clone generator-produced iterators.
> > It seems to me that the ability to back up and that of snapshotting
> > are somewhat independent.
> Backing up suggests a strictly limited buffer; cloning suggests a
Unless you need to provide "unlimited undo", yes, but that's a harder
problem anyway (needing different architecture).
> > may be just because it's the one case for which I happened to
> > stumble on some use cases in production (apart from "undoing", which
> > isn't too bad to handle in other ways anyway).
> I'd like to hear more about those cases, to see if they really need
> cloning (:-) or can live with a fixed limited backup capability.
I have an iterator it whose items, after an arbitrary prefix terminated by
the first empty item, are supposed to be each 'yes' or 'no'.
I need to process it with different functions depending if it has certain
proportions of 'yes'/'no' (and yet another function if it has any invalid
items) -- each of those functions needs to get the iterator from right
after that 'first empty item'.
Today, I do:
def dispatchyesno(it, any_invalid, selective_processing):
# skip the prefix
for x in it:
if not x: break
# snapshot the rest
snap = list(it)
it = iter(snap)
# count and check
yeses = noes = 0
for x in it:
if x=='yes': yeses += 1
elif x=='no': noes += 1
else: return any_invalid(snap)
total = float(yeses+noes)
if not total: raise ValueError, "sequence empty after prefix"
ratio = yeses / total
for threshold, function in selective_processing:
if ratio <= threshold: return function(snap)
raise ValueError, "no function to deal with a ratio of %s" % ratio
(yes, I could use bisect, but the number of items in selective_processing
is generally quite low so I didn't bother).
Basically, I punt and "snapshot" by making a list out of what is left of
my iterator after the prefix. That may be the best I can do in some cases,
but in others it's a waste. (Oh well, at least infinite iterators are not a
consideration here, since I do need to exhaust the iterator to get the
ratio:-). What I plan to do if this becomes a serious problem in the
future is add something like an optional 'clone=None' argument so I
if clone is None:
snap = list(it)
it = iter(snap)
else: snap = clone(it)
instead of what I have hardwired now. But, I _would_ like to just do, e.g.:
try: snap = it.clone()
snap = list(it)
it = iter(snap)
using some standardized protocol for "easily clonable iterators" rather
than requiring such awareness of the issue on the caller's part.
> I think a standard backup wrapper would be a useful thing to have
> (maybe in itertools?); since generator functions can't be cloned, I'm
> going to push back on the need for cloning for now until I see a lot
> more non-toy evidence.
Very reasonable, sure. I suspect the discussion of backup wrapper
is best moved to another thread, given this msg is so long and there
are all the usual finicky details to nail down....
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