[Python-Dev] copying of itertools iterators

[Raymond Hettinger]

On Apr 1, 2010, at 4:20 PM, Andrew Svetlov wrote:

> using of copy.copy for simple iterators is forbidden
> 
>>>> import copy
>>>> copy.copy(iter([1, 2, 3]))
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
>  File "/home/andrew/projects/py3k/Lib/copy.py", line 96, in copy
>    return _reconstruct(x, rv, 0)
>  File "/home/andrew/projects/py3k/Lib/copy.py", line 284, in _reconstruct
>    y = callable(*args)
>  File "/home/andrew/projects/py3k/Lib/copyreg.py", line 88, in __newobj__
>    return cls.__new__(cls, *args)
> TypeError: object.__new__(list_iterator) is not safe, use
> list_iterator.__new__()
> 
> That behavior is safe and clean.
> But it's possible to copy iterator objects returned by itertools functions:
> 
>>>> i = itertools.chain([1, 2], [3, 4, 5])
>>>> i.__next__()
> 1
>>>> j = copy.copy(i)
>>>> j.__next__()
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
> StopIteration
>>>> i.__next__()
> 2
> 
> Looks like itertools object should be protected from usage like that.
> Folks, what are you think about?
> 

I find it hard to get excited about this.
It doesn't seem to have been a problem in the real world
(no complaints, bug reports, or feature requests).

The tee() itertool is the official way to split-out an iterator
stream -- it is also copyable with copy.copy().

The itertools.count() function is also copyable.
Running copy.copy() on other itertools  is currently undefined
(though I may add copy support to itertools.repeat() and
the combinatoric functions).

Also, I seems to me the copy.copy() is itself not very bright
about what it tries to copy and in giving clear messages
about whether or not it successfully made a copy.


Raymond