How to concat column Y to column X and replicate values Z in pandas dataframe?

Question

I have a pandas DataFrame with three columns:

     X    Y    Z
0    1    4    True
1    2    5    True
2    3    6    False

How do I make it so that I have two columns X and Z with values:

    X    Z
0   1    True
1   2    True
2   3    False
3   4    True
4   5    True
5   6    False

Answer 1

you can melt:

In [41]: df.melt(id_vars="Z", value_vars=["X", "Y"], value_name="XY")[["XY", "Z"]]
Out[41]:
   XY      Z
0   1   True
1   2   True
2   3  False
3   4   True
4   5   True
5   6  False

• identifier variable is "Z": it will be repeated as necessary against value variables...
• ...which are X and Y
• name X and Y's together column to "XY", and select that and "Z" at the end

(you can chain .rename(columns={"XY": "X"}) if you want that column to be named X again.)

Answer 2

Another possible solution, based on pandas.concat:

pd.concat([df[['X','Z']], df[['Y','Z']].rename({'Y': 'X'}, axis=1)])

Output:

   X      Z
0  1   True
1  2   True
2  3  False
0  4   True
1  5   True
2  6  False