[Python] Setting slice of column to list of values on polars dataframe

In the code below I'm creating a polars- and a pandas dataframe with identical data. I want to select a set of rows based on a condition on column A, then update the corresponding rows for column C. I've included how I would do this with the pandas dataframe, but I'm coming up short on how to get this working with polars. The closest I've gotten is by using when-then-otherwise, but I'm unable to use anything other than a single value in then.

import pandas as pd
import polars as pl

df_pd = pd.DataFrame({'A': ['x', 'x', 'x', 'x', 'y', 'y', 'y', 'y'],
'B': [1, 1, 2, 2, 1, 1, 2, 2],
'C': [1, 2, 3, 4, 5, 6, 7, 8]})

df_pl = pl.DataFrame({'A': ['x', 'x', 'x', 'x', 'y', 'y', 'y', 'y'],
'B': [1, 1, 2, 2, 1, 1, 2, 2],
'C': [1, 2, 3, 4, 5, 6, 7, 8]})

df_pd.loc[df_pd['A'] == 'x', 'C'] = [-1, -2, -3, -4]

df_pl ???

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