writable iterators?

[Mel]

Steven D'Aprano wrote:

> On Wed, 22 Jun 2011 15:28:23 -0400, Neal Becker wrote:
> 
>> AFAICT, the python iterator concept only supports readable iterators,
>> not write. Is this true?
>> 
>> for example:
>> 
>> for e in sequence:
>>   do something that reads e
>>   e = blah # will do nothing
>> 
>> I believe this is not a limitation on the for loop, but a limitation on
>> the python iterator concept.  Is this correct?
> 
> Have you tried it? "e = blah" certainly does not "do nothing", regardless
> of whether you are in a for loop or not. It binds the name e to the value
> blah.
> 
>>>> seq = [1, 2]
>>>> for e in seq:
> ...     print(e)
> ...     e = 42
> ...     print(e)
> ...
> 1
> 42
> 2
> 42
> 
> 
> I *guess* that what you mean by "writable iterators" is that rebinding e
> should change seq in place, i.e. you would expect that seq should now
> equal [42, 42]. Is that what you mean? It's not clear.
> 
> Fortunately, that's not how it works, and far from being a "limitation",
> it would be *disastrous* if iterables worked that way. I can't imagine
> how many bugs would occur from people reassigning to the loop variable,
> forgetting that it had a side-effect of also reassigning to the iterable.
> Fortunately, Python is not that badly designed.

And for an iterator like

def things():
    yield 1
    yield 11
    yield 4
    yield 9

I don't know what it could even mean.

	Mel.