On Fri, 18 Jan 2013 09:10:34 +0000, Steven D'Aprano wrote: > On Fri, 18 Jan 2013 03:38:08 +0000, Dan Sommers wrote:
>> This, or something like this, is very old: >> >> sentinel = object() >> class Magic: >> def __init__(self, value): >> self.value = value >> def __call__(self, value=sentinel): >> if value != sentinel: >> self.value = value >> return self.value > > There's not really any magic in that :-) Well, not any more, no, but this one has stuck in my mind because of the magic it revealed the first time I saw it. It was definitely one of those Aha! moments for me. > Better to use "if value is not sentinel" rather than != because the > caller might provide a custom object that compares equal to sentinel. > > Also you should name it SENTINEL, or even _SENTINEL, to indicate that it > is (1) a constant, and (2) a private variable. Both fair points. > I believe that C++ calls it a "functor", not to be confused with what > Haskell calls a functor, which is completely different. But we like overloaded terminology! ;-) Dan -- http://mail.python.org/mailman/listinfo/python-list
