Why doesn't yield return individual values during each loop constructing this defaultdict?

What you say in your own answer is true. I just wanted to make sure you understand that this fact you have uncovered would be equally true of your first code example if you collected each value into a list and then printed that list using a single print statement. yield doesn't have anything to do with the issue you're seeing. I expect that you already know this, but I wanted to point it out in case someone reading this later on might think that this is a problem being introduced by using yield. It is not.

To see this, you can change your second example to print the yielded value immediately. That way, you're doing the same thing in both examples...printing the next value as soon as it is generated. If you do that, you get the same result for both versions of your code.

Here's a full set of code to demonstrate this:

from collections import defaultdict

lis = [[1, 2], [2, 1], [3, 0], [2, 1], [1, 1]]

res = defaultdict(int)
for i, j in lis:
    res[i] += j
    print(res.items())

def g(lis: list):
    res = defaultdict(int)
    for i, j in lis:
        res[i] += j
        yield res.items()

for v in g(lis):
    # Print the next generated value
    print(v)

Result:

dict_items([(1, 2)])
dict_items([(1, 2), (2, 1)])
dict_items([(1, 2), (2, 1), (3, 0)])
dict_items([(1, 2), (2, 2), (3, 0)])
dict_items([(1, 3), (2, 2), (3, 0)])
dict_items([(1, 2)])
dict_items([(1, 2), (2, 1)])
dict_items([(1, 2), (2, 1), (3, 0)])
dict_items([(1, 2), (2, 2), (3, 0)])
dict_items([(1, 3), (2, 2), (3, 0)])

Well, I find the problem.

Since defaultdict is a changeable object.

So I need to copy the res (no need to deepcopy, since items are tuples, which are unchangeable)

from collections import defaultdict
from copy import copy

li = [[1, 2], [2, 1], [3, 0], [2, 1], [1, 1]]


def g(lis: list):
    res = defaultdict(int)
    for i, j in lis:
        res[i] += j
        yield copy(res)


print([x.items() for x in g(li)])

or directly return .items()

from collections import defaultdict
from copy import copy

li = [[1, 2], [2, 1], [3, 0], [2, 1], [1, 1]]


def g(lis: list):
    res = defaultdict(int)
    for i, j in lis:
        res[i] += j
        yield copy(list(res.items()))
        # reason to add list() is TypeError: cannot pickle 'dict_items' object


print(*g(li))

Or, use list() to new an object

def g(lis: list):
    res = defaultdict(int)
    for i, j in lis:
        res[i] += j
        yield list(res.items())