[Python] In Polars, how do you create a group counter / group ID?

How do you get a group_id column like this, grouping by columns col1 and col2 ?

col1 col2 group_id
A Z 1
A Y 2
A Z 1
B Z 3

based on such a DataFrame :

df = pl.DataFrame({
'col1': ['A', 'A', 'A', 'B'],
'col2': ['Z', 'Y', 'Z', 'Z']}
)


In other words, I'm looking for polars equivalent to R data.table .GRP (df[, .GRP, by = .(col1, col2)]). Thanks !

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