check if list of lists and iterate accordingly in python

Question

I am having to deal with either lists or lists of lists. I find that my current method is not very efficient as I have to define two separate methods for doing things and effectively write them out twice. Is there a more efficient way to check if incoming data is a list or list of lists and then proceed accordingly (I usually want to act on each item of incoming data and output a list or list of lists that matches structure of the incoming data). Here's an example of me dealing with it:

if not any(isinstance(item, list) for item in keys):
    userStrings = []
    for key in keys[index]:
        userStrings[index].extend(item.GetUserStrings().GetValues(key))
else:
    userStrings = [[] for i in range(len(rhObjects))]
    for index, item in enumerate(rhObjects):
        for key in keys[index]:
            userStrings[index].extend(item.GetUserStrings().GetValues(key))

What if i encounter something even more complex where its a list of lists of lists etc. This method will not work...ideas?

Thanks!

This doesn't seem to be clear enough. Let's start over. Imagine that I have a list of lists, a list, or even a list of lists of lists of objects:

_list = [["X","Y","Z"],["X","Y","Z",["X","Y","Z"]],["X","Y","Z"]]
_list2 = ["X","Y","Z"]
_list3 = [["X","Y","Z"],["X","Y","Z"]]

All I want to do is perform a simple function on each object from these lists (append "A" to each object) or create a whole new object but in a fashion that matches structure of the original input list. Example functions:

   def my_function(obj):
       return obj + "A"
   def my_function2(obj):
       return newObj

DESIRED OUTPUT:

_newList = [["XA","YA","ZA"],["XA","YA","ZA",["XA","YA","ZA"]],["XA","YA","ZA"]]
_newList2 = ["XA","YA","ZA"]
_newList3 = [["XA","YA","ZA"],["XA","YA","ZA"]]

or if used my_function2:

_newList = [["newObj","newObj","newObj"],["newObj","newObj","newObj",["newObj","newObj","newObj"]],["newObj","newObj","newObj"]]

Is there a way to make this like the user below suggested with two functions? One recursive? I am a total noob to recursion and the first approach that you saw on top was my way of checking for lists within the list, but its really not flexible. All/any help is appreciated!

Jerome Anthony · Accepted Answer · 2014-12-02 00:39:42Z

2

Your logic should be in a separate method, and you should use recursion.

def process_list(_list):
    new_list = []
    for x in _list:
        if is_list(x):
            new_list.append(process_list(x))
        else:
            new_list.append(map(lambda y: y + "A",x))
    return new_list

def is_list(l):
    return type(l) == types.ListType

_list = [["X","Y","Z"],["X","Y","Z",["X","Y","Z"]],["X","Y","Z"]]
print(process_list(_list))
print(_list)

The code is just to demo the recursion. Hope you get it and construct your code likewise.

edited Dec 2, 2014 at 0:39

answered Dec 1, 2014 at 0:06

Jerome Anthony

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3 Comments

Jerome Anthony Over a year ago

My code is just a sketch up to show you could use recursion to inspect lists of lists. I cannot provide a code without knowing your logic. Checkout the link I provided for recursion if you want to lean about it more.

konrad Over a year ago

would you be able to demonstrate it on the example i added to the question? something really simple with strings but lists would vary from really simple to more complex. I just want to see how the functions need to be set up because when i tried your sketch it failed (infinite loop i guess)

konrad Over a year ago

This method works because strings are iterable. What if my data is a list of integers? map() fails on objects that are not iterable and I am getting an error. Is there an alternative approach to matching a data structure than map()? Thank you!

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