[Python] How do join/map a polars dataframe to a dict?

I have a polars dataframe, and a dictionary. I want to map a column in the dataframe to the keys of the dictionary, and then add the corresponding values as a new column.

import polars as pl

my_dict = {
'a': 1,
'b': 2,
}

my_df = pl.DataFrame({'region': ['a', 'b', 'a']})


┌────────┐
│ region │
│ --- │
│ str │
╞════════╡
│ a │
│ b │
│ a │
└────────┘


I want to end up with:

pl.DataFrame({'region': ['a', 'b', 'a'], 'city': [1, 2, 1]})


┌────────┬──────┐
│ region ┆ city │
│ --- ┆ --- │
│ str ┆ i64 │
╞════════╪══════╡
│ a ┆ 1 │
│ b ┆ 2 │
│ a ┆ 1 │
└────────┴──────┘


How can I do this?

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