import polars as pl
from polars import col

df = pl.DataFrame({"a": [1, 2]})
df = df.select(pl.repeat(None, 1).append(col("a")))
# shape: (3, 1)
# ┌────────┐
# │ repeat │
# │ ---    │
# │ i64    │
# ╞════════╡
# │ null   │ <-- chunk 1
# │ 1      │ <-- chunk 2
# │ 2      │
# └────────┘

df.select(col("repeat").rechunk()).n_chunks()
# 1

df.with_columns(col("repeat").rechunk()).n_chunks()
# 2