Fill pandas column forward iteratively, but without using iteration?

Question

I have a pandas data frame with a column where a condition is met based on other elements in the data frame (not shown). Additionally, I have a column that extends the validness one row further with the following rule:

If a valid row is followed directly by ExtendsValid, that row is also valid, even if the underlying valid condition doesnt apply. Continue filling valid forward as long as ExtendsValid is 1

I have illustrated the result in column "FinalValid" (desired result. Doesnt have to be a new column, can also fill Valid forward). Note that rows 8 and 9 in the example also become valid. Also note that row 13 does NOT result in FinalValid, because you need a preceding valid row. Preceding valid row can be Valid or an extended valid row.

So far if I had a problem like that I did a cumbersome multi-step process:

1. Create a new column for when "Valid" or "ExtendsValid" is true
2. Create a new column marking the start point for each "sub-series" (a consecutive set of ones)
3. Number each sub-series
4. fillna using "group by" for each sub series

I can provide sample code but I am really looking for a totally different, more efficient approach, which of course must be non-iterating as well.

Any ideas would be welcome.

#	Valid	ExtendsValid	FinalValid
1	0	0	0
2	1	0	1
3	0	0	0
4	0	0	0
5	1	0	1
6	0	0	0
7	1	0	1
8	0	1	1
9	0	1	1
10	0	0	0
11	1	0	1
12	0	0	0
13	0	1	0
14	0	0	0

Answer 1

IIUC, you want to ffill the 1s only if there is an uninterrupted series of 1s starting on Valid and eventually continuing on ExtendsValid.

For this you can use a groupby.cummin:

df['FinalValid'] = (
 (df['Valid']|df['ExtendsValid'])
 .groupby(df['Valid'].cumsum())
 .cummin()
 )

Output:

NB. I slightly modified the input on row 3 to better illustrate the logic.

     #  Valid  ExtendsValid  FinalValid
0    1      0             0           0
1    2      1             0           1
2    3      0             0           0
3    4      0             1           0
4    5      1             0           1
5    6      0             0           0
6    7      1             0           1
7    8      0             1           1
8    9      0             1           1
9   10      0             0           0
10  11      1             0           1
11  12      0             0           0