pola-rs/polars · error · TypeError

reduce_balanced() of empty iterable

Error message

reduce_balanced() of empty iterable

What it means

TypeError raised by reduce_balanced: the input iterable was empty, so there is no initial value to seed the balanced-tree reduction. It mirrors Python's built-in reduce behavior of refusing to reduce an empty sequence without an initializer.

Source

Thrown at py-polars/src/polars/_utils/reduce_balanced.py:13

from collections.abc import Callable, Iterable
from typing import TypeVar

T = TypeVar("T")


def reduce_balanced(function: Callable[[T, T], T], iterable: Iterable[T]) -> T:
    """Applies a reduction in a balanced tree pattern."""
    values = list(iterable)

    if not values:
        msg = "reduce_balanced() of empty iterable"
        raise TypeError(msg)

    if len(values) == 1:
        return values.pop()

    stack = [(0, len(values))]

    i = 0

    while i < len(stack):
        offset, length = stack[i]
        half = -(length // -2)

        if length > 3:
            stack.append((offset + half, length - half))

        if length > 2:
            stack.append((offset, half))

View on GitHub (pinned to 9b5d73fd00)

Solutions

1. Pass a non-empty iterable to reduce_balanced, or handle the empty case before calling.

Example fix

if items: reduce_balanced(items)

When it happens

Trigger: reduce_balanced called with an empty iterable.

Common situations: See trigger scenarios.

AI-assisted analysis of pola-rs/polars@9b5d73fd00 (2026-08-19). Data as JSON: /api/errors/118c5827e8557602. Report an issue: GitHub.