How to replace None in the List with previous value

You can use the function accumulate() and the operator or:

from itertools import accumulate

list(accumulate(lst, lambda x, y: y or x))
# [None, None, 1, 2, 2, 2, 3, 4, 4, 5, 5, 5]

In this solution you take the element y and the previous element x and compare them using the operator or. If y is None you take the previous element x; otherweise, you take y. If both are None you get None.

You can take advantage of lists being mutable

x =[None, None, 1, 2, None, None, 3, 4, None, 5, None, None]
for i,e in enumerate(x[:-1], 1):
    if x[i] is None:
        x[i] = x[i-1]
print(x)

output

[None, None, 1, 2, 2, 2, 3, 4, 4, 5, 5, 5]

In Python 3.8 or higher you can do this using the assignment operator:

def none_replace(ls):
    p = None
    return [p:=e if e is not None else p for e in ls]