Is there an iter which accepts a complex sentinel?

Question

I use iter with two arguments and was wondering if there is an equivalent which would accept a more complex sentinel?

As an example, in the code below

# returns digits 1 to 10 upon subsequently calling .create(), 
# then returns 'END' each time afterwards
class MyClass:
    def __init__(self):
        self.counter = 0
    def create(self):
        self.counter += 1
        if self.counter > 10:
            return 'END'
        else:
            return self.counter

c = MyClass()
for i in iter(c.create, 'END'):
    print(i)

the iteration ends upon getting 'END'. I would like to have it end after getting, say, a total of two 'END' (not necessarily one after the other - the code above will generate only 'END' after the first ten calls but one can imagine a case where they are interlaced with other values).

In essence, I am looking for a way to use a more complex sentinel. Is there such a concept?

I know I can resort to the obvious-but-ugly code mentioned in a previous question, hovewer @HappyLeapSecond answer is so elegant I would like to keep the spirit (I went though itertools but none of the available methods seem to do the job)

jonrsharpe · Accepted Answer · 2015-07-14T13:05:04.777

You could use itertools.takewhile with a stateful predicate. For example:

>>> from itertools import takewhile
>>> def create_predicate(func, max_count):
    """Return False only when func evaluates True for the max_count time."""
    def predicate(elem):
        if func(elem):
            predicate.count += 1
        if predicate.count == max_count:
            return False
        return True
    predicate.count = 0
    return predicate

>>> list(takewhile(create_predicate(lambda elem: elem % 3 == 0, 3), range(1, 20)))
[1, 2, 3, 4, 5, 6, 7, 8]

In the example above, create_predicate(lambda elem: elem % 3 == 0, 3) creates a complex predicate function that will stop the iteration on the third multiple of three. In your case,

I would like to have it end after getting, say, a total of two 'END'

you could use create_predicate(lambda elem: elem == 'END', 2).

score 0 · Answer 2 · answered Jul 14 '15 at 13:15

It could be done using a generator:

class MyClass:
    def __init__(self):
        self.counter = 0
    def create(self, func, number):
        while number:
            self.counter += 1
            if func(self.counter):
                yield 'END'
                number -= 1
            else:
                yield self.counter

Here's how it works:

>>> def foo(n):
    return n % 3 == 0
>>> c = MyClass()
>>> for i in c.create(foo, 3):
    print(i)

1
2
END
4
5
END
7
8
END

I think this keeps things fairly simple (possibly too simple for your use case but your example is quite simple).

Is there an iter which accepts a complex sentinel?

2 Answers2