Dangerous behavior of list(generator)

[M.-A. Lemburg]

exarkun at twistedmatrix.com wrote:
> On 08:45 am, tjreedy at udel.edu wrote:
>> Tom Machinski wrote:
>>> In most cases, `list(generator)` works as expected. Thus,
>>> `list(<generator expression>)` is generally equivalent to `[<generator
>>> expression>]`.
>>>
>>> Here's a minimal case where this equivalence breaks, causing a serious
>>> and hard-to-detect bug in a program:
>>>
>>>   >>> def sit(): raise StopIteration()
>>
>> 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.
> 
> What's with this view, exactly?  Is it just that it's hard to implement
> the more desirable behavior?

I'm not exactly sure what you're asking for.

The StopIteration exception originated as part of the for-loop
protocol. Later on it was generalized to apply to generators
as well.

The reason for using an exception is simple: raising and catching
exceptions is fast at C level and since the machinery for
communicating exceptions up the call stack was already there
(and doesn't interfere with the regular return values), this
was a convenient method to let the upper call levels know
that an iteration has ended (e.g. a for-loop 4 levels up the
stack).

I'm not sure whether that answers your question, but it's the
reason for things being as they are :-)

-- 
Marc-Andre Lemburg
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