[Python-ideas] sentinel_exception argument to `iter`

[Andrew Barnert]

On Feb 6, 2014, at 22:03, Terry Reedy <tjreedy at udel.edu> wrote:

> On 2/6/2014 11:15 PM, Terry Reedy wrote:
>>> On Fri, Feb 7, 2014 at 1:36 PM, Terry Reedy
> 
>>>> def __next__(self):
>>>>   try:
>>>>     x = self.func()
>>>>   except Exception as exc:
>>>>     if isinstance(exc, self.sentinel):
>>>>       raise StopIteration from None
>>         else:
>>           raise
> 
> I just realized that the above is unnecessarily complicated because the expression that follows 'except' is not limited to a builtin exception class name or tuple thereof. (I have never before had reason to dynamically determine the exception to be caught.) So, using a third parameter, replace the 5 lines with 2.
> 
>    except self.stop_exception:
>        raise StopIteration from None

Except that you don't have a stop_exception, you have a sentinel, which can be either an object or an exception type.

I'm actually not sure whether it's legal to use, say, 0 or "" as the except expression. In recent 3.4 builds, it seems to be accepted, and to never catch anything. So, if that's guaranteed by the language, it's just a simple typo to fix and your simplified implementation works perfectly.

> 
>>>>   if x == self.sentinel:
>>>>     raise StopIteration
>>>>   else:
>>>>     return x
> 
> -- 
> Terry Jan Reedy
> 
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