# How to make a flat list out of a list of lists

Given a list of lists `t`

,

`flat_list = [item for sublist in t for item in sublist]`

which means:

`flat_list = []for sublist in t: for item in sublist: flat_list.append(item)`

is faster than the shortcuts posted so far. (`t`

is the list to flatten.)

Here is the corresponding function:

`def flatten(t): return [item for sublist in t for item in sublist]`

As evidence, you can use the `timeit`

module in the standard library:

`$ python -mtimeit -s't=[[1,2,3],[4,5,6], [7], [8,9]]*99' '[item for sublist in t for item in sublist]'10000 loops, best of 3: 143 usec per loop$ python -mtimeit -s't=[[1,2,3],[4,5,6], [7], [8,9]]*99' 'sum(t, [])'1000 loops, best of 3: 969 usec per loop$ python -mtimeit -s't=[[1,2,3],[4,5,6], [7], [8,9]]*99' 'reduce(lambda x,y: x+y,t)'1000 loops, best of 3: 1.1 msec per loop`

Explanation: the shortcuts based on `+`

(including the implied use in `sum`

) are, of necessity, `O(T**2)`

when there are T sublists -- as the intermediate result list keeps getting longer, at each step a new intermediate result list object gets allocated, and all the items in the previous intermediate result must be copied over (as well as a few new ones added at the end). So, for simplicity and without actual loss of generality, say you have T sublists of k items each: the first k items are copied back and forth T-1 times, the second k items T-2 times, and so on; total number of copies is k times the sum of x for x from 1 to T excluded, i.e., `k * (T**2)/2`

.

The list comprehension just generates one list, once, and copies each item over (from its original place of residence to the result list) also exactly once.

You can use `itertools.chain()`

:

`import itertoolslist2d = [[1,2,3], [4,5,6], [7], [8,9]]merged = list(itertools.chain(*list2d))`

Or you can use `itertools.chain.from_iterable()`

which doesn't require unpacking the list with the `*`

operator:

`merged = list(itertools.chain.from_iterable(list2d))`

**Note from the author**: This is inefficient. But fun, because monoids are awesome. It's not appropriate for production Python code.

`>>> l = [[1, 2, 3], [4, 5, 6], [7], [8, 9]]>>> sum(l, [])[1, 2, 3, 4, 5, 6, 7, 8, 9]`

This just sums the elements of iterable passed in the first argument, treating second argument as the initial value of the sum (if not given, `0`

is used instead and this case will give you an error).

Because you are summing nested lists, you actually get `[1,3]+[2,4]`

as a result of `sum([[1,3],[2,4]],[])`

, which is equal to `[1,3,2,4]`

.

Note that only works on lists of lists. For lists of lists of lists, you'll need another solution.