[Python] Assigning to a subset of a Dataframe (with a selection or other method) in python...

In Pandas I can do the following:

data = pd.DataFrame(
{
"era": ["01", "01", "02", "02", "03", "10"],
"pred1": [1, 2, 3, 4, 5,6],
"pred2": [2,4,5,6,7,8],
"pred3": [3,5,6,8,9,1],
"something_else": [5,4,3,67,5,4],
})
pred_cols = ["pred1", "pred2", "pred3"]
ERA_COL = "era"
DOWNSAMPLE_CROSS_VAL = 10

test_split = ['01', '02', '10']
test_split_index = data[ERA_COL].isin(test_split)
downsampled_train_split_index = train_split_index[test_split_index].index[:OWNSAMPLE_CROSS_VAL]

data.loc[test_split_index, "pred1"] = somefunction()["another_column"]


How can I achieve the same in Polars? I tried to do some data.filter(****) = somefunction()["another_column"], but the filter output is not assignable with Polars.

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