CODEWITHSUNDEEP
pythonrecursiongenerator
Benjamin Kozuch
Benjamin Kozuch

Reputation: 150

Why is this generator (using recursion) not producing intuitive results

I've written a simple generator function that takes a list that could have sub-lists and tries to flatten the list:

so [1, [2, 3], 4, [5, [6, 7], 8]] should produce 1,2,3,4,5,6,7,8

If I just want to print out the values (not a generator) it looks like this and this works:

#  Code A
def flatten_list_of_lists(my_list):
    for element in my_list:
        if isinstance(element, list):
            flatten_list_of_lists(element)
        else:
            print(element)

my_list = [1, [2, 3], 4, [5, [6, 7], 8]]
flatten_list_of_lists(my_list)

And that prints out 1,2,3,4,5,6,7,8 as expected

However, when I change the code to this:

#  Code B
def flatten_list_of_lists(my_list):
    for element in my_list:
        if isinstance(element, list):
            flatten_list_of_lists(element)
        else:
            yield element

for i in flatten_list_of_lists(my_list):
    print(i)

which is just switching over the print to a yield, the program just prints out 1,4.

I'll paste code below that actually works. But I'm wondering why the previous code doesnt work? If Code A 'prints' out the numbers correctly, why wouldnt Code B 'yield' the numbers correctly?

Seems like I have a fundamental misunderstanding of how generators work with recursion.

This code actually works:

#  Code C
def flatten_list_of_lists_v2(my_list):
    for element in my_list:
        if isinstance(element, list):
            for sub_element in flatten_list_of_lists_v2(element):
                yield sub_element
        else:
            yield element

l = []
for element in flatten_list_of_lists_v2(my_list):
    print(element)

And that prints out 1,2,3,4,5,6,7,8

Just a little background, I just finished watching this video: https://www.youtube.com/watch?v=LelQTPiH3f4

and in there he explains when you're designing your generators, just put a print where you want to yield and see if you get the right results and then just switch the print to a yield. So I guess his advice doesnt work in all circumstances, I just want to understand why.

Upvotes: 0

Views: 51

Answers (1)

Mulan
Mulan

Reputation: 135227

A simple mistake -

def flatten_list_of_lists(my_list):
    for element in my_list:
        if isinstance(element, list):
            # add yield from
            yield from flatten_list_of_lists(element)
        else:
            # yield, not print
            yield element

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

for e in flatten_list_of_lists(my_list):
  print(e)

Output

1
2
3
4
5
6
7
8

Upvotes: 6

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