check_transformers_unfitted_stateless¶

def check_transformers_unfitted_stateless(transformer, X: pl.DataFrame) -> None:
    """Check stateless transformers transform deterministically across fits.

    Every transformer requires ``fit()`` before ``transform()`` (BaseActualTransformer
    always calls ``check_is_fitted``). For a stateless transformer
    (``observation_horizon == 0``) fitting carries no information beyond the
    schema, so two independent fits on the same data must yield identical
    transform output.

    Parameters
    ----------
    transformer : BaseActualTransformer
        Unfitted transformer expected to be stateless.
    X : pl.DataFrame
        Test data

    Raises
    ------
    AssertionError
        If a stateless transformer produces different output across fits

    """
    transformer1 = clone(transformer)
    transformer1.fit(X)

    # Only exercise genuinely stateless transformers.
    if transformer1.observation_horizon != 0:
        return

    X_trans1 = transformer1.transform(X)

    transformer2 = clone(transformer)
    transformer2.fit(X)
    X_trans2 = transformer2.transform(X)

    assert_frame_equal(X_trans1, X_trans2, rel_tol=1e-6, abs_tol=1e-8)