How to unpivot dataframe in python? [duplicate]-CodePudding

I have dataframe in pivot format but I want to make it in better way.

import pandas as pd
d = { 'year': [2019,2020,2021,2022], 'cat1': ['a','a','b','b'], 'cat2': ['c1','c2','c3','c4'],'value': [1,2,300,400]}
df = pd.DataFrame(data=d)
df.pivot(index=['year','cat1'], columns='cat2', values='value').reset_index()

output

    year    cat1  c1    c2    c3     c4
0   2019    a    1.0    NaN  NaN    NaN
1   2020    a    NaN    1.0  NaN    NaN
2   2021    b    NaN    NaN  300.0  NaN
3   2022    b    NaN    NaN  NaN    300.0

output required like this--

    year    cat1    cat2    value
0   2019    a       c1      1.0
1   2020    a       c2      1.0
2   2021    b       c3      300.0
3   2022    b       c4      300.0

CodePudding user response：

Use DataFrame.stack():

df = df.set_index(['year', 'cat1']).stack().reset_index()
df.columns = ['year', 'cat1', 'cat2', 'value']
df


   year cat1 cat2  value
0  2019    a   c1    1.0
1  2020    a   c2    1.0
2  2021    b   c3  300.0
3  2022    b   c4  300.0