Flatten a list in python

Question

I have a list like this:

[[(video1,4)], [(video2,5),(video3,8)], [(video1,5)], [(video5, 7), (video6,9)]...]

each item in this list may con

Answer 1

There is a very simple way of doing this with list comprehensions. This example has been documented in the python documentation here

>>> # flatten a list using a listcomp with two 'for'
>>> vec = [[1,2,3], [4,5,6], [7,8,9]]
>>> [num for elem in vec for num in elem]
[1, 2, 3, 4, 5, 6, 7, 8, 9]

Here is the solution that you would want to implement. As per your example, this is the simplest solution

In [59]: your_list = [[('video1',4)], [('video2',5),('video3',8)], [('video1',5)], [('video5', 7), ('video6',9)]]

In [60]: improved_list = [num for elem in your_list for num in elem]

In [61]: improved_list
Out[61]: 
[('video1', 4),
 ('video2', 5),
 ('video3', 8),
 ('video1', 5),
 ('video5', 7),
 ('video6', 9)]

Answer 2

Try this:

from itertools import chain 

c = set()
reqs = 0
for vid, number in chain(*your_list): 
    c.add(vid)
    reqs += number 

Also see related post Flattening a shallow list in Python.

There should be negligible performance increase from using chain.from_iterable(list) rather than chain(*list), but it's true that the former looks cleaner.

Answer 3

to extract all tuples from a data structure ...

def get_tups(y):
    z = []
    if y:
        for x in y:
            if isinstance(x, tuple):
                z.append(x)
            else:
                z.extend(get_tups(x))
    return z

maybe ...

Answer 4

To flatten one level, you can use itertools.chain.from_iterable():

flattened_list = itertools.chain.from_iterable(my_list)

Answer 5

If this list is singly-nested (list of lists) you can do this, which I use a lot:

flat_list = sum(list_of_lists, [])

This works due to the fact that sum simply adds up the lists, and adding up lists works in python as expected :)

Note: This is inefficient and some say unreadable.

Answer 6

If you just want to flatten the list, just use itertools.chain.from_iterable: http://docs.python.org/library/itertools.html#itertools.chain.from_iterable