Pandas DataFrame stack multiple column values into single column without changing one of the columns

Question

I'm trying to reshape a data frame in a certain way.

This is the data frame I have,

col_1	col_2
One	Roxanne
Two	Ghina
Roxanne	Leila
Ghina	George
Three	Rock
Four	Rock

I'd like to reshape the dataframe such that it looks like this:

col_3
Roxanne
Ghina
Leila
George
Rock
Rock
One
Two
Three
Four

How would I go about doing this? (without changing col_2 values)

Answer 1

For expected ouput join both columns with removed matched values from col_1 by col_2:

df = pd.concat([df.col_2, df.loc[~df.col_1.isin(df.col_2), 'col_1']]).to_frame('col_3')
print (df)
     col_3
0  Roxanne
1    Ghina
2    Leila
3   George
4     Rock
5     Rock
0      One
1      Two
4    Three
5     Four