Joining a set of ordered-integer yielding Python iterators

Question

Here is a seemingly simple problem: given a list of iterators that yield sequences of integers in ascending order, write a concise generator that yields only the integers th

Answer 1

If these are really long (or even infinite) sequences, and you don't want to load everything into a set in advance, you can implement this with a 1-item lookahead on each iterator.

EndOfIter = object() # Sentinel value

class PeekableIterator(object):
    def __init__(self, it):
        self.it = it
        self._peek = None
        self.next() # pump iterator to get first value

    def __iter__(self): return self

    def next(self):
        cur = self._peek
        if cur is EndOfIter:
            raise StopIteration()

        try:
            self._peek = self.it.next()
        except StopIteration:
            self._peek = EndOfIter
        return cur

    def peek(self): 
        return self._peek


def contained_in_all(seqs):
   if not seqs: return   # No items
   iterators = [PeekableIterator(iter(seq)) for seq in seqs]
   first, rest = iterators[0], iterators[1:]

   for item in first:
       candidates = list(rest)
       while candidates:
           if any(c.peek() is EndOfIter for c in candidates): return  # Exhausted an iterator
           candidates = [c for c in candidates if c.peek() < item]
           for c in candidates: c.next()

       # Out of loop if first item in remaining iterator are all >= item.
       if all(it.peek() == item for it in rest):
           yield item

Usage:

>>> print list(contained_in_all(postings))
[100, 322]