Is there a shortcut to make a simple list out of a list of lists in Python?

I can do it in a `for`

loop, but is there some cool "one-liner"?

I tried it with `functools.reduce()`

:

```
from functools import reduce
l = [[1, 2, 3], [4, 5, 6], [7], [8, 9]]
reduce(lambda x, y: x.extend(y), l)
```

But I get this error:

```
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 1, in <lambda>
AttributeError: 'NoneType' object has no attribute 'extend'
```

## Best Answer

Given a list of lists

`t`

,which means:

is faster than the shortcuts posted so far. (

`t`

is the list to flatten.)Here is the corresponding function:

As evidence, you can use the

`timeit`

module in the standard library: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.