from datetime import date
import polars as pl

df = pl.DataFrame({
    "a": [date(2024, 1, 1), date(2024, 2, 1)],
    "b": [date(2024, 1, 2), date(2024, 2, 2)],
})
# shape: (2, 2)
# ┌────────────┬────────────┐
# │ a          ┆ b          │
# │ ---        ┆ ---        │
# │ date       ┆ date       │
# ╞════════════╪════════════╡
# │ 2024-01-01 ┆ 2024-01-02 │
# │ 2024-02-01 ┆ 2024-02-02 │
# └────────────┴────────────┘

# Regular mean: works
df.select(
    pl.all().mean().name.suffix("_mean"),
)
# shape: (1, 2)
# ┌─────────────────────┬─────────────────────┐
# │ a_mean              ┆ b_mean              │
# │ ---                 ┆ ---                 │
# │ datetime[ms]        ┆ datetime[ms]        │
# ╞═════════════════════╪═════════════════════╡
# │ 2024-01-16 12:00:00 ┆ 2024-01-17 12:00:00 │
# └─────────────────────┴─────────────────────┘

# Mean horizontal: error
df.select(
    pl.mean_horizontal("a", "b").alias("mean")
)
# polars.exceptions.InvalidOperationError: 'horizontal_mean' expects numeric expressions, found "a" (dtype=date)