Flattening a list refers to the procedure of unpacking nested lists this list. This means, if we have a list [“a”, [“ba”, “bb”], “c”], then the flattened version of this list would look like: [“a”, “ba”, “bb”, “c”]. The flatten operation can be implemented in several different ways.
All of the following code snippets have been tested with Python 3.3.
Our running example is the following list:
list_of_lists = ["a", ["ba", "bb"], ["ca"]]
The least verbose one is to use chain from the itertools module (since 2.6):
import itertools # works on a number of lists that are given as separate parameters l1 = list(itertools.chain(*list_of_lists)) # works on a single "2D" list l2 = list(itertools.chain.from_iterable(list_of_lists))
Clearly, this should be the preferred way of implementing flatten in Python, because that is what it actually does – in one, short line.
Compared to this, the following code looks rather hacky. Note that reduce was a built-in function in earlier versions of Python.
import functools l3 = functools.reduce(lambda x,y: x + y if isinstance(y, list) else x + [y], list_of_lists, )
If all elements of list_of_list were lists, then the abbreviated form of this is
l4 = sum([["a"], ["ba", "bb"], ["ca"]],)
There are some quite fine-grained timing analyses available in the Stackoverflow questions that deal with this question, for instance by cdleary.
-  (one out of many) stackoverflow question about this problem