iter
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From: daryn on 9 Aug 2010 12:11 I'm just playing around with the iter function and I realize that I can use the iterator returned by it long after the original object has any name bound to it. Example: >>>a=[1,2,3,4] >>>b=iter(a) >>>b.next() 1 >>>a[1]=99 >>>a[3]=101 >>>del a >>>b.next() 99 >>>b.next() 3 >>>b.next() 101 it seems as if the original object is never being garbage collected even though there is no name bound to it. Does the name bound to the iterator object count as a reference to the original object for garbage collection purposes? Is there some way to retrieve/manipulate the original object via the iterator? Just trying to understand how this all works. -thanks for any help you can give daryn
From: Peter Otten on 9 Aug 2010 12:31 daryn wrote: > I'm just playing around with the iter function and I realize that I > can use the iterator returned by it long after the original object has > any name bound to it. Example: > >>>>a=[1,2,3,4] >>>>b=iter(a) >>>>b.next() > 1 >>>>a[1]=99 >>>>a[3]=101 >>>>del a >>>>b.next() > 99 >>>>b.next() > 3 >>>>b.next() > 101 > > it seems as if the original object is never being garbage collected > even though there is no name bound to it. Does the name bound to the > iterator object count as a reference to the original object for > garbage collection purposes? The listiterator object internally holds a reference to the list, but doesn't make it available to Python code. > Is there some way to retrieve/manipulate > the original object via the iterator? Not without dirty tricks (ctypes). > Just trying to understand how this all works. If you can read C look here: http://svn.python.org/view/python/branches/py3k/Objects/listobject.c?revision=81032&view=markup Peter
From: Terry Reedy on 9 Aug 2010 13:11 On 8/9/2010 12:11 PM, daryn wrote: > I'm just playing around with the iter function and I realize that I > can use the iterator returned by it long after the original object has > any name bound to it. Example: > >>>> a=[1,2,3,4] >>>> b=iter(a) >>>> b.next() > 1 >>>> a[1]=99 Changing a list while iterating through it is possible, sometimes useful, but error prone, especially with insert or delete. Changing a dict while iterating through it is prohibited since the iteration order depends on the exact internal structure. That in turn depends on the history of additions and deletions. >>>> a[3]=101 >>>> del a >>>> b.next() > 99 >>>> b.next() > 3 >>>> b.next() > 101 > > it seems as if the original object is never being garbage collected > even though there is no name bound to it. The fact that CPython currently deletes some things immediately is a current implementation detail, subject to change. > Does the name bound to the > iterator object count as a reference to the original object for > garbage collection purposes? Not quite. The iterator obviously has to have an internal reference, which amount to the almost the same thing. However, if you put the iterator in a list and deleted the name binding, both the iterator and list are still kept around. > Is there some way to retrieve/manipulate > the original object via the iterator? If you do dir(b), you will only see the standard __xx__ methods, most of which are inherited. Iterators can be written to expose an underlying object, if there is one, but some do not have one and this is not part of the simple iterator protocol. Hence builtin iterators for builtins do not do so. If one actually needed such lookup, this wrapper should work. class myiter(): def __init__(self, ob): self.ob = ob self.__itnext__ = iter(ob).__next__ def __iter__(self): return self def __next__(self): return self.__itnext__() it = myiter([1,2,3]) print (it.ob, list(it)) # [1, 2, 3] [1, 2, 3] > Just trying to understand how this all works. Keep experimenting. Python makes is so easy, especially with an edit window such as with IDLE and a Run command. I initially tried not defining myiter.__next__ and instead wrote: self.__next__ = iter(ob).__next__ but that does not work because special __xx__ methods are typically looked up on the class, not the instance. I know that but was not completely sure about this case. -- Terry Jan Reedy
From: Steven D'Aprano on 10 Aug 2010 00:12 On Mon, 09 Aug 2010 09:11:37 -0700, daryn wrote: > I'm just playing around with the iter function and I realize that I can > use the iterator returned by it long after the original object has any > name bound to it. Yes, the same as everything else in Python. Iterators aren't unique here. >>> a = [1,2,3] >>> b = [None, a, None] >>> id(a) == id(b[1]) # Same object, not a copy? True >>> del a >>> print b [None, [1, 2, 3], None] > it seems as if the original object is never being garbage collected even > though there is no name bound to it. Of course not. That would be a Bad Thing if Python garbage collected an object while other objects were still using it. Can you say "core dump"? > Does the name bound to the > iterator object count as a reference to the original object for garbage > collection purposes? No, but the iterator itself does. The technique is called *reference* counting, not "name counting". Each name is a reference, but not every reference is a name. > Is there some way to retrieve/manipulate the > original object via the iterator? Depends on the iterator. For the standard iterator created by iter(), I don't think so. But for a custom iterator type, there can be if you want: class MyIter(object): """Quick and dirty iterator.""" def __init__(self, data): self.data = data self.i = 0 def __iter__(self): return self def next(self): try: o = self.data[self.i] except IndexError: raise StopIteration self.i += 1 return o >>> it = MyIter([1,2,3,4]) >>> it.next() 1 >>> del it.data[1:3] >>> it.next() 4 -- Steven
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