[Python] How to create a rank on two columns in python polars?

Suppose we have this dataframe in polars (python):

import polars as pl
df = pl.DataFrame(
{
"era": ["01", "01", "02", "02", "03", "03"],
"pred": [3,5,6,8,9,1]
}
)


I can create a rank/row_number based on one column, like:

df.with_columns(rn = pl.col("era").rank("ordinal"))


But if I want to do it based on two columns, it is not working:

df.with_columns(rn = pl.col(["era","pred"]).rank("ordinal"))


I get this error message:

ComputeError: The name: 'rn' passed to `LazyFrame.with_columns` is duplicate
Error originated just after this operation:
DF ["era", "pred"]; PROJECT */2 COLUMNS; SELECTION: "None"


Any suggestions on how to do this?

Continue reading...