pola-rs/polars · error · TypeError

" " comparison not supported for LazyFrame objects

Error message

"{operator!r}" comparison not supported for LazyFrame objects

What it means

LazyFrames are lazy/query objects, not row data, so Polars forbids element-wise comparison operators (==, !=, <, >, <=, >=) on them. All rich comparisons on LazyFrame raise this TypeError because comparing two query plans is undefined. Comparing a LazyFrame is almost always a mistake where the user meant to compare collected DataFrames or to build an expression inside select/filter.

Solutions

1. Collect both sides and compare DataFrames: `lf.collect() == lf2.collect()`
2. If comparing plans, compare `lf.explain()` strings or serialized plans instead
3. For row logic, use expression comparisons inside `filter`/`select` (e.g. `lf.select(pl.col('a') == 1)`)
4. Replace `assert lf == x` in tests with `assert_frame_equal` from polars.testing

Example fix

// before
assert lf == lf2

// after
from polars.testing import assert_frame_equal
assert_frame_equal(lf.collect(), lf2.collect())

if isinstance(obj, pl.LazyFrame):
    obj = obj.collect()  # materialize before comparing

def is_lazyframe(obj) -> bool:
    return isinstance(obj, pl.LazyFrame)

try:
    result = lf == other
except TypeError as e:
    if "comparison not supported for LazyFrame" in str(e):
        result = lf.collect() == other.collect()
    else:
        raise

When it happens

Trigger: Any direct comparison: lf == other_lf, lf != x, lf > 5, sorting a list of LazyFrames, or using lf in a context that calls __eq__ (e.g. `lf in [lf1, lf2]`, set/dict usage, `assert lf == expected`).

Common situations: Unit tests asserting two lazy frames are equal; using a LazyFrame in an `in` check or as a dict key; code refactored from DataFrame to LazyFrame; accidentally comparing a LazyFrame to a DataFrame.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• arr.to_struct() got a str instead of a list. hint: pass
• cannot call `map_groups` when filtering groups with `having`
• cannot call `map_groups` when grouping by an expression
• cannot call `map_groups` when grouping by named expressions
• cannot describe a LazyFrame that has no columns

AI-assisted analysis of pola-rs/polars@fe841f959e (2026-09-18). Data as JSON: /api/errors/a2e811a605cf0708. Report an issue: GitHub.

Appendix: source

Thrown at py-polars/src/polars/lazyframe/frame.py:727

        """
        issue_warning(
            "determining the width of a LazyFrame requires resolving its schema,"
            " which is a potentially expensive operation. Use `LazyFrame.collect_schema().len()`"
            " to get the width without this warning.",
            category=PerformanceWarning,
        )
        return self.collect_schema().len()

    def __bool__(self) -> NoReturn:
        msg = (
            "the truth value of a LazyFrame is ambiguous"
            "\n\nLazyFrames cannot be used in boolean context with and/or/not operators."
        )
        raise TypeError(msg)

    def _comparison_error(self, operator: str) -> NoReturn:
        msg = f'"{operator!r}" comparison not supported for LazyFrame objects'
        raise TypeError(msg)

    def __eq__(self, other: object) -> NoReturn:
        self._comparison_error("==")

    def __ne__(self, other: object) -> NoReturn:
        self._comparison_error("!=")

    def __gt__(self, other: Any) -> NoReturn:
        self._comparison_error(">")

    def __lt__(self, other: Any) -> NoReturn:
        self._comparison_error("<")

    def __ge__(self, other: Any) -> NoReturn:
        self._comparison_error(">=")

    def __le__(self, other: Any) -> NoReturn:
        self._comparison_error("<=")

View on GitHub (pinned to fe841f959e)