pola-rs/polars · error · TypeError

from_lazyframe.resolver(): expected instance of LazyFrameRes

Error message

from_lazyframe.resolver(): expected instance of LazyFrameResolver, got: {resolver = } {type(resolver) = }

What it means

LazyFrame.from_lazyframe(...) with a resolver argument requires `resolver` to be an actual LazyFrameResolver instance. Any other object (None, a string, a callable, a wrapped proxy) raises this TypeError, echoing the value and its type to help identify what was passed.

Source

Thrown at py-polars/src/polars/lazyframe/frame.py:393

            .lazy()
            ._ldf
        )

    @classmethod
    def from_lazyframe_resolver(cls, resolver: LazyFrameResolver) -> LazyFrame:
        """
        Create a LazyFrame from a LazyFrame resolver.

        .. warning::
                This functionality is considered **unstable**. It may be changed
                at any point without it being considered a breaking change.
        """
        if not isinstance(resolver, LazyFrameResolver):
            msg = (
                "from_lazyframe.resolver(): expected instance of LazyFrameResolver, "
                f"got: {resolver = } {type(resolver) = }"
            )
            raise TypeError(msg)

        return wrap_ldf(PyLazyFrame.from_lazyframe_resolver(resolver))

    @classmethod
    def _from_pyldf(cls, ldf: PyLazyFrame) -> LazyFrame:
        self = cls.__new__(cls)
        self._ldf = ldf
        return self

    def _with_height_column(self) -> LazyFrame:
        """Add a private dummy column that preserves the input height."""
        return self.with_columns(F.lit({}, dtype=Struct({})).alias(_HEIGHT_COLUMN))

    def _drop_height_column(self) -> LazyFrame:
        """Drop the dummy column added by `_with_height_column()`."""
        return self.drop(_HEIGHT_COLUMN)

    def _height_preserving(self, op: Callable[[LazyFrame], LazyFrame]) -> LazyFrame:

View on GitHub (pinned to 68506541d2)

Solutions

Construct and pass a LazyFrameResolver instance (e.g. LazyFrameResolver(...) or the factory that produces one), not a class or callable.
Check `isinstance(resolver, LazyFrameResolver)` before the call; inspect the printed type(resolver) in the message to see what you actually passed.
Ensure a single polars installation is imported (no duplicated/aliased polars packages) so the isinstance check matches.

Example fix

// before
lf2 = LazyFrame.from_lazyframe(lf, resolver=my_resolver_fn)
// after
resolver = LazyFrameResolver(my_resolver_fn)
lf2 = LazyFrame.from_lazyframe(lf, resolver=resolver)

Defensive patterns

Strategy: type-guard

Validate before calling

from polars.lazyframe.frame import LazyFrameResolver
if not isinstance(resolver, LazyFrameResolver):
    raise TypeError(f"resolver must be LazyFrameResolver, got {type(resolver)}")

Type guard

def is_lazyframe_resolver(resolver) -> bool:
    from polars.lazyframe.frame import LazyFrameResolver
    return isinstance(resolver, LazyFrameResolver)

Try / catch

try:
    out = LazyFrame.from_lazyframe(lf, resolver=resolver)
except TypeError as e:
    if "expected instance of LazyFrameResolver" in str(e):
        resolver = LazyFrameResolver(resolver)
        out = LazyFrame.from_lazyframe(lf, resolver=resolver)
    else:
        raise

Prevention

Pass instances, not classes or callables
Import LazyFrameResolver from a single canonical module path
Pin one polars version to avoid isinstance failing across duplicate installs
Assert resolver type in tests around from_lazyframe

When it happens

Trigger: Calling LazyFrame.from_lazyframe(lf, resolver=<x>) where x is not an instance of polars.lazyframe.frame.LazyFrameResolver — e.g. passing a plain function, a string name of a resolver, None, or a resolver class instead of an instance.

Common situations: Confusing the resolver class with an instance; passing an older-style callback; mixing polars versions where the resolver type lives in a different module so isinstance fails across duplicate imports.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of pola-rs/polars@68506541d2 (2026-09-10). Data as JSON: /api/errors/55aa78868f8cb4cb. Report an issue: GitHub.