Pandas: Multiple columns into one column

Update

pandas has a built in method for this stack which does what you want see the other answer.

This was my first answer before I knew about stack many years ago:

In [227]:

df = pd.DataFrame({'Column 1':['A', 'B', 'C', 'D'],'Column 2':['E', 'F', 'G', 'H']})
df
Out[227]:
  Column 1 Column 2
0        A        E
1        B        F
2        C        G
3        D        H

[4 rows x 2 columns]

In [228]:

df['Column 1'].append(df['Column 2']).reset_index(drop=True)
Out[228]:
0    A
1    B
2    C
3    D
4    E
5    F
6    G
7    H
dtype: object

The trick is to use stack()

df.stack().reset_index()
    
   level_0   level_1  0
0        0  Column 1  A
1        0  Column 2  E
2        1  Column 1  B
3        1  Column 2  F
4        2  Column 1  C
5        2  Column 2  G
6        3  Column 1  D
7        3  Column 2  H