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. It can work in Python because exceptions are objects like anything else and can be passed as arguments. No new parameter is needed. We only need to add 'or raises' to the two-parameter iter definition "In the second form, the callable is called until it returns the sentinel." to get "In the second form, the callable is called until it returns or raises the sentinel." With this version of the proposal, your pop example would work as written. The two-parameter form return a 'callable_iterator' object. Its __next__ method in Python might look like (untested) def __next__(self): x = self.func() if x == self.sentinel: raise StopIteration else: return x The only change needed for the added behavior is to wrap the function call and change an exception matching the sentinel to StopIteration. def __next__(self): try: x = self.func() except Exception as exc: if isinstance(exc, self.sentinel): raise StopIteration from None if x == self.sentinel: raise StopIteration else: return x I do something similar to this in my custom function tester and it is really handy to be able to pass in an expected exception as the expected 'output'. For example, my 'test table' for an int divide function might contain these input and expected output pairs: (2,0), ZeroDevisionError (2,1), 2 (2,2), 1. 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, rarer still to write one that can also raise an instance of the same exception class. Also, using an exception-returning function in two-parameter iter is tricky because exceptions inherit the default equality comparison of only equal by identity.
ValueError('high') == ValueError('high') False e=ValueError('high') e == e True
The following tested example of code that would be broken by the proposal could well be the first ever written. Warning: it is so twisted and awful that it might to read it. --- from random import random e=ValueError('high value') def crazy(): x = random() if x > .999: return e elif x < .001: raise ValueError('low value') else: return x try: for x in iter(crazy, e): pass else: print(e.args[0]) except ValueError as exc: print(exc.args[0]) # prints 'high value' or 'low value' in less than a second -- Terry Jan Reedy