[Python] How to add a new level to JSON output using Polars in Python?

I'm using Polars to process a DataFrame so I can save it as JSON. I know I can use the method .write_json(), however, I would like to add a new level to the JSON.

My current approach:

import polars as pl


df = pl.DataFrame({
"id": [1, 2, 3, 4, 5],
"variable1": [15, 25, 5, 10, 20],
"variable2": [40, 30, 50, 10, 20],
})
(
df.write_json()
)


Current output:

'[{"id":1,"variable1":15,"variable2":40},{"id":2,"variable1":25,"variable2":30},{"id":3,"variable1":5,"variable2":50},{"id":4,"variable1":10,"variable2":10},{"id":5,"variable1":20,"variable2":20}]'


But I would like to save it in this way, with the "Befs" key, so each "Befs" contains every record of the DataFrame.

Desired output:

{
"Befs": [
{
"ID ": 1,
"variable1": 15,
"variable2": 40
},
{
"ID ": 2,
"variable1": 25,
"variable2": 30
}

]
}


I have tried using .pl.struct() , but my attemps make no sense:

(
df
.select(
pl.struct(
pl.lit("Bef").alias("Bef"),
pl.col("id"),
pl.col("variable1"),
pl.col("variable2")
)
)
.write_json()
)

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