Summary

• normalize SequentialFeatureSelector CV handling with check_cv so iterable/generator inputs are reused safely
• document that iterables are materialized once and extend tests to cover generator and integer CV inputs

Resolves #55.

Reproduction (pre-fix)

from sklearn.datasets import make_classification
from sklearn.feature_selection import SequentialFeatureSelector
from sklearn.neighbors import KNeighborsClassifier
from sklearn.model_selection import LeaveOneGroupOut
import numpy as np

X, y = make_classification(random_state=0)

groups = np.zeros_like(y, dtype=int)
groups[y.size//2:] = 1

cv = LeaveOneGroupOut()
splits = cv.split(X, y, groups=groups)

clf = KNeighborsClassifier(n_neighbors=5)
seq = SequentialFeatureSelector(clf, n_features_to_select=5, scoring='accuracy', cv=splits)
seq.fit(X, y)

IndexError: list index out of range

Testing

• `LD_LIBRARY_PATH=/nix/store/wffgswxkp55xi14jy63rjsnfvl2qvmxy-gcc-14.3.0-lib/lib:/nix/store/4wdz42ns29ys6fm1xak68bnp51nxhd2s-zlib-1.3.1/lib:/nix/store/gh2dd8vimringn726ndall19gbm77prj-openblas-0.3.30/lib:/nix/store/y6zrr7dfg051mz4dpjvaldy4g9cy6wmq-lapack-3/lib nix shell nixpkgs#python310 nixpkgs#python310Packages.pip --command python3.10 - <<'PY'
  import importlib.machinery
  import importlib.util
  import sys
  from pathlib import Path

import pytest
import sklearn
import sklearn.feature_selection

name = "sklearn.feature_selection._sequential"
path = Path("/workspace/scikit-learn/sklearn/feature_selection/_sequential.py")
loader = importlib.machinery.SourceFileLoader(name, str(path))
spec = importlib.util.spec_from_loader(name, loader)
module = importlib.util.module_from_spec(spec)
loader.exec_module(module)
sys.modules[name] = module
sklearn.feature_selection._sequential = module
sklearn.feature_selection.SequentialFeatureSelector = module.SequentialFeatureSelector

sys.exit(
pytest.main(
[
"--import-mode=importlib",
"scikit-learn/sklearn/feature_selection/tests/test_sequential.py::test_sfs_supports_iterable_cv_generator",
"scikit-learn/sklearn/feature_selection/tests/test_sequential.py::test_sfs_baseline_cv_int_runs",
]
)
)
PY`

• nix shell nixpkgs#python310 nixpkgs#python310Packages.pip --command flake8 sklearn/feature_selection/_sequential.py sklearn/feature_selection/tests/test_sequential.py