Dangerous behavior of list(generator)

[exarkun at twistedmatrix.com]

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()
>
>>What's with this view, exactly?  Is it just that it's hard to 
>>implement
>>the more desirable behavior?
>
>What is that "more desirable behaviour"? That StopIteration is used to
>signal that Python should stop iterating except when you want it to be
>ignored? Unfortunately, yes, it's quite hard to implement "do what the
>caller actually wants, not what he asked for" behaviour -- and even if 
>it
>were possible, it goes against the grain of the Zen of Python.
>
>If you've ever had to debug faulty "Do What I Mean" software, you'd see
>this as a good thing.

I have plenty of experience developing and debugging software, Steven. 
Your argument is specious, as it presupposes that only two possibilities 
exist: the current behavior of some kind of magical faerie land 
behavior.

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).

You chopped out all the sections of this thread which discussed the more 
desirable behavior.  You can go back and read them in earlier messages 
if you need to be reminded.  I'm not talking about anything beyond 
what's already been raised.

I'm pretty sure I know the answer to my question, though - it's hard to 
implement, so it's not implemented.

Jean-Paul