Nested generators and yield from?

Question

I have an nested object, a , and want to apply some type of filter on each of its sub-containers:

a = [[1, 2], [3, 4, 5, 6], [7, 7, 8]]

A function to keep the nested structure but filter to only even elements would look like:

def nested_filter(obj):
    res = []
    for sub_obj in obj:
        even = [i for i in sub_obj if i % 2 == 0]
        res.append(even)
    return res

nested_filter(a)
# [[2], [4, 6], [8]]

But I would like to create an equivalent generator that uses some form a nested yield to maintain the nested structure of a .

This, for illustration, is exactly what I do not want because it flattens a :

def nested_yield(obj):
    """Will flatten the nested structure of obj."""
    for sub_obj in obj:
        for i in sub_obj:
            if i % 2 == 0:
                yield i

list(nested_yield(a))
# [2, 4, 6, 8]

My understanding was that yield from , introduced in Python 3.3 , might allow for nesting that retains the structure of the passed object. But apparently I am misinterpreting:

def gen1(obj):
    for sub_obj in obj:
        yield from gen2(sub_obj)


def gen2(sub_obj):
    for i in sub_obj:
        if i % 2 == 0:
            yield i

list(gen1(a))
# [2, 4, 6, 8]

So, is there any way I can use two generator functions (ie not create intermediate lists) so that calling the generator would give me this:

list(nested_generator(a))
# [[2], [4, 6], [8]]

Answer 1

You want your nested_generator to return a list of sublists. In this case, you cannot get around creating temporary lists at the inner level.

So, your best option is to yield the filtered list:

>>> def nested_filter(L):
    for l in L:
        yield [i for i in l if i % 2 == 0]


>>> a = [[1, 2], [3, 4, 5, 6], [7, 7, 8]]
>>> list(nested_filter(a))
[[2], [4, 6], [8]]

If you want to use a second generator to filter your sub-list, you can convert the iterator returned to a list and yield that as well:

>>> def generator_filter(L):
    for n in L:
        if n % 2 == 0:
            yield n


>>> def nested_filter(L):
    for l in L:
        yield list(generator_filter(l))


>>> a = [[1, 2], [3, 4, 5, 6], [7, 7, 8]]
>>> list(nested_generator(a))
[[2], [4, 6], [8]]