UnPivot a Pandas Data Set with Merge

Question

How do I transform this starting data set into a flattened data set with a Python Pandas data frame?

Like this flattened data:

I tried to "stack" the data and reset the index, but this produced an undesired result.

df = xl.parse("data")
stack = df.stack(-1).reset_index(0)

Thanks in advance for your help.

Answer 1

You're looking for melt (aka "unpivot"):

In [11]: df = pd.DataFrame([["a", "b", 43, 87, 29]], columns=["N", "P", 1, 2, 3])

In [12]: pd.melt(df, id_vars=["N", "P"], value_vars=[1, 2, 3], var_name="Day")
Out[12]:
   N  P Day  value
0  a  b   1     43
1  a  b   2     87
2  a  b   3     29