[Python] How can I iterate over all columns using pl.all() in Polars?

I've written a custom function in Polars to generate a horizontal forward/backward fill list of expressions. The function accepts an iterable of expressions (or column names) to determine the order of filling. I want to to use all columns via pl.all() as default. The problem is that pl.all() returns a single expression rather than an iterable, so trying to reverse or iterate over it leads to a TypeError.

Is there a way to convert between single expressions and iterables of expressions? Any suggestions or workarounds are greatly appreciated!

Here is the function:

from typing import Iterable
from polars._typing import IntoExpr
import polars as pl

def fill_horizontal(exprs: Iterable[IntoExpr], forward: bool = True) -> list[pl.Expr]:
"""Generate a horizontal forward/backward fill list of expressions."""
# exprs = exprs or pl.all() # use all columns as default
cols = [col for col in reversed(exprs)] if forward else exprs
return [pl.coalesce(cols[i:]) for i in range(0, len(cols) - 1)]


Here is an example:

df = pl.DataFrame({
"col1": [1, None, 2],
"col2": [1, 2, None],
"col3": [None, None, 3]})
print(df)
# shape: (3, 3)
# ┌──────┬──────┬──────┐
# │ col1 ┆ col2 ┆ col3 │
# │ --- ┆ --- ┆ --- │
# │ i64 ┆ i64 ┆ i64 │
# ╞══════╪══════╪══════╡
# │ 1 ┆ 1 ┆ null │
# │ null ┆ 2 ┆ null │
# │ 2 ┆ null ┆ 3 │
# └──────┴──────┴──────┘
print('forward_fill')
print(df.with_columns(fill_horizontal(df.columns, forward=True)))
# shape: (3, 3)
# ┌──────┬──────┬──────┐
# │ col1 ┆ col2 ┆ col3 │
# │ --- ┆ --- ┆ --- │
# │ i64 ┆ i64 ┆ i64 │
# ╞══════╪══════╪══════╡
# │ 1 ┆ 1 ┆ 1 │
# │ null ┆ 2 ┆ 2 │
# │ 2 ┆ 2 ┆ 3 │
# └──────┴──────┴──────┘
print('backward_fill')
print(df.with_columns(fill_horizontal(df.columns, forward=False)))
# shape: (3, 3)
# ┌──────┬──────┬──────┐
# │ col1 ┆ col2 ┆ col3 │
# │ --- ┆ --- ┆ --- │
# │ i64 ┆ i64 ┆ i64 │
# ╞══════╪══════╪══════╡
# │ 1 ┆ 1 ┆ null │
# │ 2 ┆ 2 ┆ null │
# │ 2 ┆ 3 ┆ 3 │
# └──────┴──────┴──────┘

Continue reading...