Partial List Flattening In Python

I have a small problem where I lack quite some Python experience. Assuming a list: list=[[40, 20, 45, 40], [[20, 10, 10, 30, 45, 20], [30, 20, 20, 30]]] I want to do something I c

Solution 1:

You can create a recursive function, testing whether all the inner elements are lists. If so, recursively apply the algorithm, otherwise yield the list itself.

definner_lists(lst):
    ifall(isinstance(x, list) for x in lst):
        for inner in lst:
            for x in inner_lists(inner):
                yield x
    else:
        yield lst

>>> lst = [[40, 20, 45, 40], [[20, 10, 10, 30, 45, 20], [30, 20, 20, 30]]]
>>> list(inner_lists(lst))
[[40, 20, 45, 40], [20, 10, 10, 30, 45, 20], [30, 20, 20, 30]]

Or using yield from (Python 3.3 and later):

definner_lists(lst):
    ifall(isinstance(x, list) for x in lst):
        yieldfrom (x for inner in lst for x in inner_lists(inner))
    else:
        yield lst

Or without generators:

def inner_lists(lst):
    if all(isinstance(x, list) for x in lst):
        return [x forinnerin lst for x in inner_lists(inner)]
    else:
        return [lst]

Solution 2:

If your lists can only contains numbers or sub lists (not both of them) you can use following generator function:

In [25]: def partial_flattening(lst):
             forsubin lst:
                 if isinstance(sub[0], list):
                     yield from sub # in python 2.x use for i insub: yeild i
                 else:          
                     yieldsub
   ....:                 

In [26]: list(partial_flattening(lst))
Out[36]: [[40, 20, 45, 40], [20, 10, 10, 30, 45, 20], [30, 20, 20, 30]]

If there may be combination of both types then based on what you want to do with the numbers you can customize the function.

Or as a more concise approach you can use yield within list comprehension:

In [29]: list([(yield from i) if isinstance(i[0], list) else (yield i) for i in lst])
Out[29]: [[40, 20, 45, 40], [20, 10, 10, 30, 45, 20], [30, 20, 20, 30]]