import polars as pl

df = pl.DataFrame({"A": [[1, 2]], "B": [[3, 4]]})
df = df.with_columns(pl.all().arr.to_struct())

df.with_columns(pl.col("A").struct.rename_fields(["x", "y", "z"])).unnest("A", "B")

# shape: (1, 4)
# ┌─────┬─────┬─────────┬─────────┐
# │ x   ┆ y   ┆ field_0 ┆ field_1 │
# │ --- ┆ --- ┆ ---     ┆ ---     │
# │ i64 ┆ i64 ┆ i64     ┆ i64     │
# ╞═════╪═════╪═════════╪═════════╡
# │ 1   ┆ 2   ┆ 3       ┆ 4       │
# └─────┴─────┴─────────┴─────────┘

# A.field_1 disappears
df.with_columns(pl.col("A").struct.rename_fields(["x"])).unnest("A", "B")

# shape: (1, 3)
# ┌─────┬─────────┬─────────┐
# │ x   ┆ field_0 ┆ field_1 │
# │ --- ┆ ---     ┆ ---     │
# │ i64 ┆ i64     ┆ i64     │
# ╞═════╪═════════╪═════════╡
# │ 1   ┆ 3       ┆ 4       │
# └─────┴─────────┴─────────┘

df.with_columns(pl.col("A").struct.rename_fields([])).unnest("A", "B")

# shape: (1, 3)
# ┌──────┬─────────┬─────────┐
# │      ┆ field_0 ┆ field_1 │
# │ ---  ┆ ---     ┆ ---     │
# │ null ┆ i64     ┆ i64     │
# ╞══════╪═════════╪═════════╡
# │ null ┆ 3       ┆ 4       │
# └──────┴─────────┴─────────┘