Python-Polars merge lists of lists

Question

Hello I have a polars dataframe with a column that contains lists of lists. like this:

df = pl.DataFrame({
    "nodes": [[[1], [1,2], []], [[2]], [[1], [2]]]
})

I want to merge all these sublists into one single list like this:

shape: (3, 1)
┌───────────┐
│ nodes     │
│ ---       │
│ list[i64] │
╞═══════════╡
│ [1, 1, 2] │
│ [2]       │
│ [1, 2]    │
└───────────┘

I am pretty sure that must be doable pretty easily but I don't find the correct way in the API. Thanks for any help!

Answer 1

• pl.Expr.list.eval() and pl.element() to run Expr evaluation within outer list.
• pl.Expr.explode() to explode inner lists.
• pl.Expr.drop_nulls() to drop result from exploding empty list.

df.select(
    pl.col.nodes.list.eval(pl.element().explode().drop_nulls())
)

shape: (3, 1)
┌───────────┐
│ nodes     │
│ ---       │
│ list[i64] │
╞═══════════╡
│ [1, 1, 2] │
│ [2]       │
│ [1, 2]    │
└───────────┘