Fixpoint for Iterators

fixit.py

import collections.abc as ABC
from itertools import islice, tee

class MutableIterator(ABC.Iterator):
    def __init__(self, nexts=()):
        self.nexts = list(nexts)

    def __next__(self):
        if self.nexts:
            return self.nexts.pop()
        else:
            raise StopIteration()

    def append(self, new):
        self.nexts += [new]

def fix_it(it):
    """Fixpoint for iterators."""
    self = MutableIterator()
    for i in it(self):
        self.append(i)
        yield i
        
# Example
def recursive_fibs(self):
    yield from (0, 1, 1)
    (left, right) = tee(self)
    next(right)
    yield from (x + y for (x, y) in zip(left, right))

def fib(n):
    return next(islice(fix_it(recursive_fibs), n-1, None))


# Another example
def up(n):
    def k(self):
        yield 0
        yield from (x + 1 for x in self if x < n - 1)
    return k

Try list(fix_it(up(10))) and fib(1000).