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[Python-ideas] sentinel_exception argument to `iter`

Terry Reedy tjreedy at udel.edu
Fri Feb 7 09:36:51 CET 2014 
On 2/7/2014 1:52 AM, Andrew Barnert wrote:
> 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 wrote the above with the idea that there would be a third parameter to 
provide an exception. It would have to have a private default like

class _NoException(Exception): pass

> 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.

Easy to test. The result is that a non-exception expression is 
syntactically legal, but not when an exception occurs.

 >>> try: 3
except 1: 4

3
 >>> try: 1/0
except 1: 4

Traceback (most recent call last):
   File "<pyshell#56>", line 1, in <module>
     try: 1/0
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
   File "<pyshell#56>", line 2, in <module>
     except 1: 4
TypeError: catching classes that do not inherit from BaseException is 
not allowed

-- 
Terry Jan Reedy
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