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

Question

I wonder whether there is a shortcut to make a simple list out of list of lists in Python.I can do that in a for loop, but maybe there is some cool "one-liner"? I tried it with *reduce*, but I get an error.**Code** l = [[1, 2, 3], [4, 5, 6], [7], [8, 9]] reduce(lambda x, y: x.extend(y), l) **Error message** Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 1, in <lambda> AttributeError: 'NoneType' object has no attribute 'extend'

Answer 1

There are many solutions, the closest thing to a "one liner" would be this (I would recommend still calling a flatten function instead of using that convoluted one line of code all over your file though):

def flatten(element):

return sum( ([x] if not isinstance(x, list) else flatten(x) for x in element), [] )

here is the most trivial one in my opinion, using a combination of recursion and iteration:

def flatten(inputList):

finalList = []

for item in inputList:

if isinstance(item, list):

for deepItem in flatten(item):

finalList.append(deepItem)

else:

finalList.append(item)

return finalList

Hope this helps!