Answers for "fetch two columns from dataframe"

filter dataframe by two columns

#tl;dr use the '&' opperator and not the word 'and'
print(df)
OUTPUT
  Name  Age
0  Joe   23
1  Tom   54
2  Joe   34
3  Tom   42
4  Joe   23
5  Tom   54
6  Joe   34
7  Tom   42

tom_and_42 = df[(df["Name"]=="Tom") & (df["Age"]==42)] 

print(tom_and_42)
OUTPUT
  Name  Age
3  Tom   42
7  Tom   42

df multiple columns into one column

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