inserttag.py

from itertools import groupby, count
bol = [None, "2018",None,None,None,"2018","2019","2020",None,None]
value = "Data"
def get_bol(a):
    index = []
    for i in range(len(a)-1):
        if isinstance(a[i], str):
            index.append(i)
    return index
def set_bol(a,intervals,value): 
    for i in intervals:
        a[i] = value
    return a   
def intervals(data):
    out = []
    counter = count()
    for key, group in groupby(data, key = lambda x: x-next(counter)):
        block = list(group)
        out.append(block[-1]+1)
    return out
id = intervals(get_bol(bol))

print(set_bol(bol,id,value))
#Output
# [None, '2018', 'Data', None, None, '2018', '2019', '2020', 'Data', None]