How do I flatten a list containing both values and other lists?

Question

For instance, I could have this list:

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

and I want to flatten the list like this:

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

I found some methods online for doing this, but they assume that the list doesn't contain any values outside of sub-lists, or have side effects like reversing the order of elements. What's the best way to do this?

[Edit] I got an existing question recommended, but I couldn't understand what "yield from" does in Python, as I haven't seen it before in that context.

score 1 · Answer 1 · answered May 24 '21 at 03:03

1

# initializing the data and empty list
data = [1, [2, 3, [4, 5]], 6, [[7], [8, 9]]]
flat_list = []

# function
def flatten_list(data):
    # iterating over the data
    for element in data:
        # checking for list
        if type(element) == list:
            # calling the same function with current element as new argument
            flatten_list(element)
        else:
            flat_list.append(element)

# flattening the given list
flatten_list(data)

# printing the flat_list
print(flat_list)

(See more at https://geekflare.com/flatten-list-python/)

answered May 24 '21 at 03:03

CoderCookie10

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Is there a way to modify it so that it doesn't need the global variable `flat_list`? – haley May 24 '21 at 03:05
I don't think so. But if you're looking for a smarter method, you can use itertools.chain() – CoderCookie10 May 24 '21 at 03:08
` import itertools data = [[1, 2, 3], [4, 5], [6, 7, 8, 9, 10]] flat_list = itertools.chain(*data) print(list(flat_list))` – CoderCookie10 May 24 '21 at 03:08
1

That stops working when you have a value in only 1 level of lists, like the `1` in my example `[1, [2, 3], [4, 5, [6, 7], 8]]`... “TypeError: 'int' object is not iterable“ – haley May 24 '21 at 03:11
ohh. In that case, you should use the code above. – CoderCookie10 May 24 '21 at 03:12
However, that code will be slow for it is a recursive function. – CoderCookie10 May 24 '21 at 03:13

How do I flatten a list containing both values and other lists?

1 Answers1