Fill the values between two pandas column values with same values

Question

I have a data frame like this,

df1
col1    col2
 1        A
 2        A
 3        A
 4        B
 5        A
 6        A
 7        B
 8        A
 9        A
10        A
11        C
12        C
13        A
14        A
15        C
16        A
17        C

In above data frame total number of B and C are always even. Now I want to fill all the values between two B and C with B and C.

So the final data frame should look like,

df1
col1    col2
 1        A
 2        A
 3        A
 4        B
 5        B
 6        B
 7        B
 8        A
 9        A
10        A
11        C
12        C
13        A
14        A
15        C
16        C
17        C

I could do it using a for loop, but the execution time will be huge, I am looking for some pandas shortcut / pythonic way to do it.

Answer 1

Idea is filter out consecutive B or C values, then replace all another B or C to missing values. Then forward filling missing values but keep only values same like backfilling, last replace all another values to original with Series.fillna :

for v in ['B','C']:
    m1 = df['col2'].eq(v)
    m2 = m1.ne(m1.shift()).cumsum().duplicated(keep=False)
    s = df['col2'].where(m1 & ~m2)
    ff = s.ffill()
    df['col2'] = ff.where(ff == s.bfill()).fillna(df['col2'])
print (df)
    col1 col2
0      1    A
1      2    A
2      3    A
3      4    B
4      5    B
5      6    B
6      7    B
7      8    A
8      9    A
9     10    A
10    11    C
11    12    C
12    13    A
13    14    A
14    15    C
15    16    C
16    17    C

Answer 2

You only need to select when the cumulative sum Series.cumsum is odd + Series.mask :

for l in ['B','C']:
    mask=(df.col2.eq(l).cumsum()%2)==1
    df['col2']=df['col2'].mask(mask,l)
print(df)

    col1 col2
0     1    A 
1     2    A 
2     3    A 
3     4    B 
4     5    B 
5     6    B 
6     7    B 
7     8    A 
8     9    A 
9    10    A 
10   11    C 
11   12    C 
12   13    A 
13   14    A 
14   15    C 
15   16    C 
16   17    C