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

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

>Solution :

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.)