Boolean value of generators

[Albert Hopkins]

On Thu, 2010-10-14 at 10:16 +0100, Tony wrote:
> I have been using generators for the first time and wanted to check for
> an empty result.  Naively I assumed that generators would give
> appopriate boolean values.  For example
> 
> def xx():
>   l = []
>   for x in l:
>     yield x
> 
> y = xx()
> bool(y)
> 

As people have already mentioned, generators are objects and objects
(usually) evaluate to True.

There may be times, however, that a generator may "know" that it
doesn't/isn't/won't generate any values, and so you may be able to
override boolean evaluation.  Consider this example:

        class DaysSince(object):
            def __init__(self, day):
                self.day = day
                self.today = datetime.date.today()
        
            def __nonzero__(self):
                if self.day > self.today:
                    return False
                return True
        
            def __iter__(self):
                one_day = datetime.timedelta(1)
                new_day = self.day
                while True:
                    new_day = new_day + one_day
                    if new_day <= self.today:
                        yield new_day
                    else:
                        break
        
        g1 = DaysSince(datetime.date(2010, 10, 10))
        print bool(g1)
        for day in g1:
            print day
        
        g2 = DaysSince(datetime.date(2011, 10, 10))
        print bool(g2)
        for day in g2:
            print
        day                                                                   


> True
> 2010-10-11
> 2010-10-12
> 2010-10-13
> 2010-10-14
> False