Flatten List of Lists

Python do not have in the List library function to flatten a LoL, but don’t despair here are two possible ways.
The first trick works only for pure 2 level list, as you can see :

: sum([[1],[2,3]],[])                                                                                                                                                 
[1, 2, 3]

: sum([[1],[2,3,[4,5]]],[])                                                                                                                                           
[1, 2, 3, [4, 5]]

: sum([1,[2,3]],[])                                                                                                                                                   
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-23-d56eb05286e1> in <module>
----> 1 sum([1,[2,3]],[])

TypeError: can only concatenate list (not "int") to list

the second one guard against “dismantling” a string :

def flatten(LoL):
	for el in LoL:
		if isinstance(el, collections.Iterable) and not isinstance(el, str):
			for sub in flatten(el): yield sub
		else: yield el

: list(flatten2([1,[2,3,[4,5],'abc',(6,7,'cde')]]))                                                                                                                   
: [1, 2, 3, 4, 5, 'abc', 6, 7, 'cde']

Tags: flattenlist