Given

a = [None,1,2,3,None,4,None,None]

I'd like

a = [None,1,2,3,3,4,4,4]

Currently I have brute forced it with:

def replaceNoneWithLeftmost(val):last = Noneret = []for x in val:if x is not None:ret.append(x)last = xelse:ret.append(last)return ret

Finally, I'd like to get to

a = [1,1,2,3,3,4,4,4]

by running this right to left. Currently I have

def replaceNoneWithRightmost(val):return replaceNoneWithLeftmost(val[::-1])[::-1]

I'm not fussy about inplace or create a new list, but right now this smells to me. I can't see a way to store a temporary 'last' value and use map/lambda, and nothing else is coming to mind.

IIUC, you could use itertools.accumulate to generate a forward fill:

>>> from itertools import accumulate
>>> a = [None,1,2,3,None,4,None,None]
>>> list(accumulate(a, lambda x,y: y if y is not None else x))
[None, 1, 2, 3, 3, 4, 4, 4]