pola-rs/polars · error · AssertionError

are different ( )\n[left]: \n[right]

Error message

{objects} are different ({detail})\n[left]:  {left}\n[right]: {right}

What it means

Shared helper that formats and raises the standard polars testing 'X are different (detail)' AssertionError with left/right values. It is the single raise site behind assert_series_equal / assert_frame_equal mismatch details.

Solutions

1. Read the (detail) field: 'dtype mismatch', 'string_value mismatch', 'shape mismatch', etc., and fix the flagged aspect.
2. For float data, pass check_exact=False with an appropriate abs_tol/rel_tol.
3. Align dtypes explicitly with .cast() before comparing if the difference is benign.
4. Set check_names=False when intentionally comparing series with different names.

Example fix

# before
assert_series_equal(pl.Series([0.1 + 0.2]), pl.Series([0.3]))  # fails

# after
assert_series_equal(pl.Series([0.1 + 0.2]), pl.Series([0.3]), check_exact=False, abs_tol=1e-9)

if s1.dtype != s2.dtype:
    s2 = s2.cast(s1.dtype)
assert_series_equal(s1, s2)

try:
    assert_series_equal(left, right)
except AssertionError as e:
    print(e)  # detail names the mismatched aspect
    raise

When it happens

Trigger: Any assert_series_equal/assert_frame_equal call where values, dtypes, names, shapes, or null counts differ; _assert_correct_input_type when a wrong input type is passed to a testing helper.

Common situations: Unit tests comparing floats with NaN without check_nan; dtype drift after polars upgrade (Int32 vs Int64); column name mismatches (check_names); null vs NaN comparisons.

Related errors

• are equal (but are expected not to be)
• Series are equal (but are expected not to be)
• Cannot compare two series of different lengths.
• comparing categoricals with different Categories is not…
• comparing datetimes with different units or timezones is…

AI-assisted analysis of pola-rs/polars@fe841f959e (2026-09-18). Data as JSON: /api/errors/4597bebf2d49d587. Report an issue: GitHub.

Appendix: source

Thrown at py-polars/src/polars/testing/asserts/utils.py:12

from __future__ import annotations

from typing import Any, NoReturn


def raise_assertion_error(
    objects: str, detail: str, left: Any, right: Any, *, cause: Exception | None = None
) -> NoReturn:
    """Raise a detailed assertion error."""
    __tracebackhide__ = True
    msg = f"{objects} are different ({detail})\n[left]:  {left}\n[right]: {right}"
    raise AssertionError(msg) from cause

View on GitHub (pinned to fe841f959e)