How to save an element from a list as a whole after an if-statement

Question

I want to save elements of a list as a whole in a new list.

My new task is to transform nested lists into a flat one. My plan was to take out the elements of each list and save them into a variable. And in the end to put the variables together, so that there is just a flat list at the end.

I have two problems:

The index-3 element ('99999') isn't saved as a whole in the list of the new variable. This problem is the topic of this question.
I cannot separate the nested list [66, 77, 88] from the higher list ['4', '5',[]], but this is not the topic of this question

So here is the code of the nested list and of my if statement:

nested_list = ['1', '2', '3', '99999',['4', '5',[66, 77, 88]]]
y = []
for i in nested_list:
    if type(i) != (list or tuple):
        y += i
print(y)

I want the index=3 element (the string '99999') be saved in the variable y as a whole string and not split into the single parts '9', '9', '9', '9', '9'

I want:

print(y)
['1', '2', '3', '99999']

I get:

print(y)
['1', '2', '3', '9', '9', '9', '9', '9']

I can't solve the problem with saving into a string (y = ' '), because than I get the result:

print(y)
12399999

and if I would transform the string to a list

y = list(y)

I get the same unwanted result

['1', '2', '3', '9', '9', '9', '9', '9']

I think the solution lies in the action code after the if-statement

y += i

Is there a command, which takes the whole element into y? I tried y += sum(i) but of course this didn't work, because

it adds up all numbers like int, floats, etc.., not strings
I don't need to add 9+9+9+9+9=45, but I need just the whole string in my new list y.

Possible duplicate of https://stackoverflow.com/questions/952914/how-to-make-a-flat-list-out-of-list-of-lists — Dani Mesejo, Sep 29 '19 at 16:15
You might consider using `y.append(i)` instead of `y += i`. They apparently have different behaviors. See this [answer](https://stackoverflow.com/a/2022044/11450390) — SacrificerXY, Sep 29 '19 at 16:17
Note that `type(i) != (list or tuple)` **does not** do what you think - try `isinstance(i, (list, tuple))` instead. — jonrsharpe, Sep 29 '19 at 16:22

Frank · Accepted Answer · 2019-09-29T16:33:47.527

I don't know if i understand you right:

# List comprehension:
y = [x for x in nested_list if not isinstance(x, (list, tuple))]
# gives: ['1', '2', '3', '99999']

It would be the same as:

nested_list = ['1', '2', '3', '99999',['4', '5',[66, 77, 88]]]
y = []
for i in nested_list:
    if not isinstance(i, (list or tuple)):
        y.append(i)  # This maybe your failure
print(y)

Does this help you or do you need to have all other elements also in your output list? (i only recognized your example)

Additional if you want to iterate over all and flatten the whole list:

nested_list = ['1', '2', '3', '99999',['4', '5',[66, 77, 88]]]
y = []


def flatten_list(lst):
    if isinstance(lst, (list, tuple)):
        if len(lst) == 0:
            return []
        first, rest = lst[0], lst[1:]
        return flatten_list(first) + flatten_list(rest)
    else:
        return [lst]

y = flatten_list(nested_list)

print(y)  # ['1', '2', '3', '99999', '4', '5', 66, 77, 88]

score 2 · Answer 2 · answered Sep 29 '19 at 16:22

One issue is that (list or tuple) evaluates to a boolean itself, and rather you should use isinstance for type checking

Then, try appending to the list rather than adding stings, which are iterable in themselves. For example, you could've tested [] + '9999' independently for your problem

y = []
for i in nested_list:
    if isinstance(i, (list, tuple));
        y.append(i)

How to save an element from a list as a whole after an if-statement

2 Answers2