Merging multiple columns into a single column using pandas

Question

Suppose I have a dataframe as follows

  C1 C2
0  A B
1  C NaN
2  E F
3  G H

how can I merge the two columns into one using pandas

output

  new
0  A
1  B
2  C
3  NaN
4  E
5  F
6  G
7  H

Answer 1

Use DataFrame constructor with converted DataFrame to numpy array:

df = pd.DataFrame({'new':df.values.ravel()})

Or stack with reset_index:

df = df.stack(dropna=False).reset_index(drop=True).to_frame('new')
print (df)
   new
0    A
1    B
2    C
3  NaN
4    E
5    F
6    G
7    H