[Python] iloc equivalent for selection by position and setting values?

[H G]

iloc equivalent for selection by position and setting values?

import pyarrow as pa
import pandas as pd
df = pd.DataFrame({'year': [2020, 2022, 2019, 2021],
                   'n_legs': [2, 4, 5, 100],
                   'animals': ["Flamingo", "Horse", "Brittle stars", None]})
table = pa.Table.from_pandas(df)

df.loc[df["animals"].isnull(), "animals"] = "new_value" # how do we perform
this in pyarrow?

I did open this on github, but I assume it is not the forum for queries.

Thanks