On Thu, Feb 06, 2014 at 09:36:19PM -0500, Terry Reedy wrote:
On 2/6/2014 7:10 PM, Ram Rachum wrote:
`iter` has a very cool `sentinel` argument. I suggest an additional argument `sentinel_exception`; when it's supplied, instead of waiting for a sentinel value, we wait for a sentinel exception to be raised, and then the iteration is finished.
This'll be useful to construct things like this:
my_iterator = iter(my_deque.popleft, IndexError)
What do you think?
I think this would be a great idea if simplified to reuse the current parameter.
That would be a backwards-incompatible change, for exactly the reason you give below:
It can work in Python because exceptions are objects like anything else and can be passed as arguments.
Right. And there is a big difference between *returning* an exception and *raising* an exception, which is why a new parameter (or a new function) is required. A function might legitimately return exception objects for some reason: exceptions_to_be_tested = iter( [IndexError(msg), ValueError, StopIteration, TypeError] ) def func(): # pre- or post-processing might happen return next(it) for exception in iter(func, StopIteration): # assume the exceptions are caught elsewhere raise exception With the current behaviour, that will raise IndexError and ValueError, then stop. With the suggested change in behaviour, it will raise all four exceptions. We cannot assume that an exception is never a legitimate return result from the callable. "Iterate until this exception is raised" and "iterate until this value is returned" are very different things and it is folly to treat them as the same. [...]
I consider the threat to backward compatibility, because of the added test for exceptions, theoretical rather than actual. It is very rare to write a function that returns an exception,
Rare or not, I've done it, it's allowed by the language, and it is inappropriate to conflate returning a class or instance with raising an exception. It doesn't matter whether it is rare. It is rare to write: iter(func, ({}, {})) nevertheless it would be poor design to have iter treat tuples of exactly two dicts as a special case. Exceptions are first-class values like strings, ints, and tuples containing exactly two dicts. They should be treated exactly the same as any other first-class value. -- Steven