How do I merge two python iterators?

Question

I have two iterators, a list and an itertools.count object (i.e. an infinite value generator). I would like to merge these two into a resulting ite

Answer 1

Using itertools.izip(), instead of zip() as in some of the other answers, will improve performance:

As "pydoc itertools.izip" shows: "Works like the zip() function but consumes less memory by returning an iterator instead of a list."

Itertools.izip will also work properly even if one of the iterators is infinite.

Answer 2

You can use zip as well as itertools.chain. This will only work if the first list is finite:

merge=itertools.chain(*[iter(i) for i in zip(['foo', 'bar'], itertools.count(1))])

Answer 3

You can do something that is almost exaclty what @Pramod first suggested.

def izipmerge(a, b):
  for i, j in itertools.izip(a,b):
    yield i
    yield j

The advantage of this approach is that you won't run out of memory if both a and b are infinite.

Answer 4

I prefer this other way which is much more concise:

iter = reduce(lambda x,y: itertools.chain(x,y), iters)

Answer 5

I also agree that itertools is not needed.

But why stop at 2?

  def tmerge(*iterators):
    for values in zip(*iterators):
      for value in values:
        yield value

handles any number of iterators from 0 on upwards.

UPDATE: DOH! A commenter pointed out that this won't work unless all the iterators are the same length.

The correct code is:

def tmerge(*iterators):
  empty = {}
  for values in itertools.izip_longest(*iterators, fillvalue=empty):
    for value in values:
      if value is not empty:
        yield value

and yes, I just tried it with lists of unequal length, and a list containing {}.

Answer 6

A generator will solve your problem nicely.

def imerge(a, b):
    for i, j in itertools.izip(a,b):
        yield i
        yield j