Dynamically get value from column1 and assign it to NaN values in same column, if they all have the same column2 value

Question

I have a DataFrame that looks like this:

      ID  DESCRIPTION TYPE1 TYPE2
12345678 EXAMPLENAME1 874.4   NaN
12345678 EXAMPLENAME2 854.4   NaN
12345678 EXAMPLENAME3 874.4   B-5
78978999 EXAMPLENAME2 788.8   B-9
78978999 EXAMPLENAME4 978.2   NaN
78978999 EXAMPLENAME1 288.3   NaN
92124566 EXAMPLENAME3 369.1   NaN
92124566 EXAMPLENAME3 289.1   B-3
92124566 EXAMPLENAME3 959.1   NaN

I want to get all the TYPE2 column that have the same ID with the value that is not NaN . We can assume that:

1. Only one TYPE2 row per ID will have a nonnull value.
2. TYPE2 is unique per each ID.

Final product should look like this:

      ID  DESCRIPTION TYPE1 TYPE2
12345678 EXAMPLENAME1 874.4   B-5
12345678 EXAMPLENAME2 854.4   B-5
12345678 EXAMPLENAME3 874.4   B-5
78978999 EXAMPLENAME2 788.8   B-9
78978999 EXAMPLENAME4 978.2   B-9
78978999 EXAMPLENAME1 288.3   B-9
92124566 EXAMPLENAME3 369.1   B-3
92124566 EXAMPLENAME3 289.1   B-3
92124566 EXAMPLENAME3 959.1   B-3

I've tried with ffill , but can't establish the condition to fill only when ID is the same. There are about 1,500,000 different TYPE2 and ID values, so manually establish them like df.loc[df["ID"]="12345678", "TYPE2"] = "B-5" wouldn't work.

How can I have df.loc check if the ID is the same, then grabbing the nonnull value from TYPE2 and assing it to the rest of the ID rows? Are there any other methods to get the same outcome?

Answer 1

To fill null values within each ID we must first use .groupby and then combine .bfill() and .ffill()

df['TYPE2'] = df.groupby('ID')['TYPE2'].bfill().ffill()
#result
    ID          DESCRIPTION     TYPE1   TYPE2
0   12345678    EXAMPLENAME1    874.4   B-5
1   12345678    EXAMPLENAME2    854.4   B-5
2   12345678    EXAMPLENAME3    874.4   B-5
3   78978999    EXAMPLENAME2    788.8   B-9
4   78978999    EXAMPLENAME4    978.2   B-9
5   78978999    EXAMPLENAME1    288.3   B-9
6   92124566    EXAMPLENAME3    369.1   B-3
7   92124566    EXAMPLENAME3    289.1   B-3
8   92124566    EXAMPLENAME3    959.1   B-3