Merging two dictionaries where items are interchanged

Question

Assume I've got the dictionaries:

dict1 = {'A': 1, 'B': 2, 'C' : 3}
dict2 = {'a': 4, 'b': 5, 'c' : 6}

This link suggest several ways to merge the two but all of the merges are simply concatenations. I want to merge them like a dealer shuffles cards, or like a zipper zips. What I mean by this is that once merging dict1 and dict2, the resulting dict3 should become

dict3 = {'A': 1, 'a': 4, 'B': 2, 'b': 5, 'C' : 3, 'c' : 6}

So the merge grabs elements from dict1 and dict2 in an alternating fashion. My dictionaries are in fact very large so doing it manually is not an option.

Answer 1

There is round-robin itertools recipe to select data this way.

You can use:

dict3 = dict(roundrobin(dict1.items(), dict2.items()))

output:

{'A': 1, 'a': 4, 'B': 2, 'b': 5, 'C': 3, 'c': 6}

recipe:

from itertools import cycle, islice
def roundrobin(*iterables):
    "roundrobin('ABC', 'D', 'EF') --> A D E B F C"
    # Recipe credited to George Sakkis
    num_active = len(iterables)
    nexts = cycle(iter(it).__next__ for it in iterables)
    while num_active:
        try:
            for next in nexts:
                yield next()
        except StopIteration:
            # Remove the iterator we just exhausted from the cycle.
            num_active -= 1
            nexts = cycle(islice(nexts, num_active))

You can also use more-itertools.interleave

from more_itertools import interleave

dict(interleave(dict1.items(), dict2.items()))

Answer 2

Indented for readability:

from itertools import chain
dict(
    chain.from_iterable(
        zip(dict1.items(), dict2.items())
    )
)

If your dictionaries are not guaranteed to be of equal size, adapt this solution to use itertools.zip_longest() instead of basic zip()

Answer 3

There are many things that could go wrong with this approach. What if your 2 dictionnaries don't have the same length? What if they have duplicate keys? What if later you need 3 dicts instead of 2?

Also, do you have a real use case behind this requirement? Do you really need to build a new dict with all data like this, or do you simply need to iterate over the (key, value) pairs in a specific, alternate fashion like this?

Assuming you don't have to worry about the above, if you only need to iterate, you could simply do something like that:

def iter_zip(dict1, dict2):
    for i1, i2 in zip(dict1.items(), dict2.items()):
        yield i1
        yield i2
   
dict1 = {'A': 1, 'B': 2, 'C' : 3}
dict2 = {'a': 4, 'b': 5, 'c' : 6}

# you can then do
for key, val in iter_zip(dict1, dict2):
    do_something(key, val)

And if you really need to build a new dict, it is now as simple as:

dict(iter_zip(dict1, dict2))
# {'A': 1, 'a': 4, 'B': 2, 'b': 5, 'C': 3, 'c': 6}