How can I change the original DataFrame from a group?

Question

Let's suppose I have the following DataFrame:

import pandas as pd

df = pd.DataFrame({'label': ['a', 'a', 'b', 'b', 'a', 'b', 'c', 'c', 'a', 'a'], 
                   'numbers': [1, 1, 1, 1, 1, 2, 2, 2, 2, 2],
                   'arbitrarydata': [False] * 10})

I want to assign a value to the arbitrarydata column according to the values in both of the other colums. A naive approach would be as follows:

for _, grp in df.groupby(('label', 'numbers')):
    grp.arbitrarydata = pd.np.random.rand()

Naturally, this doesn't propagate changes back to df. Is there a way to modify a group such that changes are reflected in the original DataFrame ?

Answer 1

Try using transform, e.g.:

df['arbitrarydata'] = df.groupby(('label', 'numbers')).transform(lambda x: np.random.rand())