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 iterator that will alternate yield values between the two:

>>> import itertools
>>> c = itertools.count(1)
>>> items = ['foo', 'bar']
>>> merged = imerge(items, c)  # the mythical "imerge"
>>> merged.next()
'foo'
>>> merged.next()
1
>>> merged.next()
'bar'
>>> merged.next()
2
>>> merged.next()
Traceback (most recent call last):
    ...
StopIteration

What is the simplest, most concise way to do this?

Answer 1

I'd do something like this. This will be most time and space efficient, since you won't have the overhead of zipping objects together. This will also work if both a and b are infinite.

def imerge(a, b):
    i1 = iter(a)
    i2 = iter(b)
    while True:
        try:
            yield i1.next()
            yield i2.next()
        except StopIteration:
            return

Answer 2

A generator will solve your problem nicely.

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

Answer 3

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 4

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 5

Why is itertools needed?

def imerge(a,b):
    for i,j in zip(a,b):
     yield i
     yield j

In this case at least one of a or b must be of finite length, cause zip will return a list, not an iterator. If you need an iterator as output then you can go for the Claudiu solution.

Answer 6

I'm not sure what your application is, but you might find the enumerate() function more useful.

>>> items = ['foo', 'bar', 'baz']
>>> for i, item in enumerate(items):
...  print item
...  print i
... 
foo
0
bar
1
baz
2

Answer 7

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.

Answer 8

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 9

Use izip and chain together:

>>> list(itertools.chain.from_iterable(itertools.izip(items, c))) # 2.6 only
['foo', 1, 'bar', 2]

>>> list(itertools.chain(*itertools.izip(items, c)))
['foo', 1, 'bar', 2]

Answer 10

A concise method is to use a generator expression with itertools.cycle(). It avoids creating a long chain() of tuples.

generator = (it.next() for it in itertools.cycle([i1, i2]))