Pandas - Insert blank row for each group in pandas

Question

I have a dataframe

import pandas as pd
import numpy as np
df1=pd.DataFrame({'group':[1,1,2,2,2],
             'value':[2,3,np.nan,5,4]})
df1

    group   value
0   1       2
1   1       3
2   2       NaN
3   2       5
4   2       4

I want to add a row after each group in which the value of value is NaN . The desire output is:

   group   value
0   1       2
1   1       3
2   1       NaN
3   2       NaN
4   2       5
5   2       4
6   2       NaN

In my real dataset I have a lot of groups and more columns besides value, I want all of them to be NaN in newly added row.

Thanks a lot for the help

Answer 1

concat with append

s = df1.groupby('group')
out = pd.concat([i.append({'value': np.nan}, ignore_index=True) for _, i in s])
out.group = out.group.ffill().astype(int)

apply with append^[1]

df1.groupby('group').apply(
    lambda d: d.append({'group': d.name}, ignore_index=True).astype({'group': int})
).reset_index(drop=True)

Both produce:

   group  value
0      1    2.0
1      1    3.0
2      1    NaN
3      2    NaN
4      2    5.0
5      2    4.0
6      2    NaN

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^[1] This solution brought to you by your local @piRSquared