Doing the opposite of pivot in pandas Python

Question

I'm trying to do the opposite of pivot in pandas. Wondering whether someone can provide some assistance? Trying to transform the data from the below

|Date|A|B|C|D|E |
|----|-|-|-|-|--|
|2005|1|2|3|4|50|
|2006|6|7|8|9|10|

to

|Date|X|Val|
|----|-|---|
|2005|A|1  |
|2005|B|2  |
|2005|C|3  |
etc 
|2006|A|6  |
etc
|2006|E|10 |

Thank you very much

Answer 1

Use set_index with stack:

df = df.set_index('Date').stack().reset_index(name='Val').rename(columns={'level_1':'X'})
print (df)
   Date  X  Val
0  2005  A    1
1  2005  B    2
2  2005  C    3
3  2005  D    4
4  2005  E   50
5  2006  A    6
6  2006  B    7
7  2006  C    8
8  2006  D    9
9  2006  E   10

Or melt, but there is different ordering of values:

df = df.melt('Date', var_name='X', value_name='Val')
print (df)
   Date  X  Val
0  2005  A    1
1  2006  A    6
2  2005  B    2
3  2006  B    7
4  2005  C    3
5  2006  C    8
6  2005  D    4
7  2006  D    9
8  2005  E   50
9  2006  E   10

So for same output add sort_values:

df = (df.melt('Date', var_name='X', value_name='Val')
        .sort_values(['Date','X'])
        .reset_index(drop=True))
print (df)
   Date  X  Val
0  2005  A    1
1  2005  B    2
2  2005  C    3
3  2005  D    4
4  2005  E   50
5  2006  A    6
6  2006  B    7
7  2006  C    8
8  2006  D    9
9  2006  E   10