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

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.