Dangerous behavior of list(generator)

[Lie Ryan]

On 12/14/09, exarkun at twistedmatrix.com <exarkun at twistedmatrix.com> wrote:
> On 02:50 am, lie.1296 at gmail.com wrote:
>>On 12/14/2009 9:45 AM, exarkun at twistedmatrix.com wrote:
>>>On 08:18 pm, steve at remove-this-cybersource.com.au wrote:
>>>>On Sun, 13 Dec 2009 14:35:21 +0000, exarkun wrote:
>>>>>>StopIteration is intended to be used only within the .__next__
>>>>>>method of
>>>>>>iterators. The devs know that other 'off-label' use results in the
>>>>>>inconsistency you noted, but their and my view is 'don't do that'.
>>>>>
>>>>>Which is unfortunate, because it's not that hard to get
>>>>>StopIteration
>>>>>without explicitly raising it yourself and this behavior makes it
>>>>>difficult to debug such situations.
>>>>
>>>>I can't think of any way to get StopIteration without explicitly
>>>>raising
>>>>it yourself. It's not like built-ins or common data structures
>>>>routinely
>>>>raise StopIteration. I don't think I've *ever* seen a StopIteration
>>>>that
>>>>I didn't raise myself.
>>>
>>>Call next on an iterator. For example: iter(()).next()
>>
>>.next() is not meant to be called directly
>
> Doesn't matter.  Sometimes it makes sense to call it directly.  And I
> was just giving an example of a way to get StopIteration raised without
> doing it yourself - which is what Steve said he couldn't think of.
>>>
>>>I'm surprised to hear you say that the magical faerie land behavior
>>>isn't desirable either, though. I'd love a tool that did what I
>>>wanted,
>>>not what I asked. The only serious argument against this, I think, is
>>>that it is beyond our current ability to create (and so anyone
>>>claiming
>>>to be able to do it is probably mistaken).
>>
>>In your world, this is what happens:
>> >>> list = [a, b, c]
>> >>> # print list
>> >>> print list
>>["a", "b", "c"]
>> >>> # make a copy of list
>> >>> alist = list(llst) # oops a mistype
>> >>> alist = alist - "]" + ", "d"]"
>> >>> print alist
>>["a", "b", "c", "d"]
>> >>> alist[:6] + "i", + alist[6:]
>> >>> print alist
>>["a", "i", "b", "c", "d"]
>> >>> print alist
>> >>> # hearing the sound of my deskjet printer...
>> >>> C:\fikle.text.write(alist)
>> >>> print open("C:\file.txt").read()
>><h1>a</h1>
>><ul>
>><li>i</li>
>><li>b</li>
>><li>c d</li>
>> >>> # great, exactly what I needed
>
> I don't understand the point of this code listing, sorry.  I suspect you
> didn't completely understand the magical faerie land I was describing -
> where all your programs would work, no matter what mistakes you made
> while writing them.

Exactly, that's what's happening. It just works. It knows that when I
said alist[:6] + "i", + alist[6:] ; I want to insert "i" between the
sixth character of the textual representation of the list. It knows to
find the correct variable when I made a typo. It correctly guess that
I want to print to a paper instead of to screen. It knows that when I
wrote to C:\path.write(), it knows I wanted a HTML output in that
specific format. It just works (TM), whatever mistakes I made. That's
what you wanted, right?