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+import warnings
+
+import numpy as np
+import pytest
+
+import sklearn
+from sklearn.datasets import load_iris, make_blobs, make_circles
+from sklearn.decomposition import PCA, KernelPCA
+from sklearn.exceptions import NotFittedError
+from sklearn.linear_model import Perceptron
+from sklearn.metrics.pairwise import rbf_kernel
+from sklearn.model_selection import GridSearchCV
+from sklearn.pipeline import Pipeline
+from sklearn.preprocessing import StandardScaler
+from sklearn.utils._testing import (
+    assert_allclose,
+    assert_array_almost_equal,
+    assert_array_equal,
+)
+from sklearn.utils.fixes import CSR_CONTAINERS
+from sklearn.utils.validation import _check_psd_eigenvalues
+
+
+def test_kernel_pca(global_random_seed):
+    """Nominal test for all solvers and all known kernels + a custom one
+
+    It tests
+     - that fit_transform is equivalent to fit+transform
+     - that the shapes of transforms and inverse transforms are correct
+    """
+    rng = np.random.RandomState(global_random_seed)
+    X_fit = rng.random_sample((5, 4))
+    X_pred = rng.random_sample((2, 4))
+
+    def histogram(x, y, **kwargs):
+        # Histogram kernel implemented as a callable.
+        assert kwargs == {}  # no kernel_params that we didn't ask for
+        return np.minimum(x, y).sum()
+
+    for eigen_solver in ("auto", "dense", "arpack", "randomized"):
+        for kernel in ("linear", "rbf", "poly", histogram):
+            # histogram kernel produces singular matrix inside linalg.solve
+            # XXX use a least-squares approximation?
+            inv = not callable(kernel)
+
+            # transform fit data
+            kpca = KernelPCA(
+                4, kernel=kernel, eigen_solver=eigen_solver, fit_inverse_transform=inv
+            )
+            X_fit_transformed = kpca.fit_transform(X_fit)
+            X_fit_transformed2 = kpca.fit(X_fit).transform(X_fit)
+            assert_array_almost_equal(
+                np.abs(X_fit_transformed), np.abs(X_fit_transformed2)
+            )
+
+            # non-regression test: previously, gamma would be 0 by default,
+            # forcing all eigenvalues to 0 under the poly kernel
+            assert X_fit_transformed.size != 0
+
+            # transform new data
+            X_pred_transformed = kpca.transform(X_pred)
+            assert X_pred_transformed.shape[1] == X_fit_transformed.shape[1]
+
+            # inverse transform
+            if inv:
+                X_pred2 = kpca.inverse_transform(X_pred_transformed)
+                assert X_pred2.shape == X_pred.shape
+
+
+def test_kernel_pca_invalid_parameters():
+    """Check that kPCA raises an error if the parameters are invalid
+
+    Tests fitting inverse transform with a precomputed kernel raises a
+    ValueError.
+    """
+    estimator = KernelPCA(
+        n_components=10, fit_inverse_transform=True, kernel="precomputed"
+    )
+    err_ms = "Cannot fit_inverse_transform with a precomputed kernel"
+    with pytest.raises(ValueError, match=err_ms):
+        estimator.fit(np.random.randn(10, 10))
+
+
+def test_kernel_pca_consistent_transform(global_random_seed):
+    """Check robustness to mutations in the original training array
+
+    Test that after fitting a kPCA model, it stays independent of any
+    mutation of the values of the original data object by relying on an
+    internal copy.
+    """
+    # X_fit_ needs to retain the old, unmodified copy of X
+    state = np.random.RandomState(global_random_seed)
+    X = state.rand(10, 10)
+    kpca = KernelPCA(random_state=state).fit(X)
+    transformed1 = kpca.transform(X)
+
+    X_copy = X.copy()
+    X[:, 0] = 666
+    transformed2 = kpca.transform(X_copy)
+    assert_array_almost_equal(transformed1, transformed2)
+
+
+def test_kernel_pca_deterministic_output(global_random_seed):
+    """Test that Kernel PCA produces deterministic output
+
+    Tests that the same inputs and random state produce the same output.
+    """
+    rng = np.random.RandomState(global_random_seed)
+    X = rng.rand(10, 10)
+    eigen_solver = ("arpack", "dense")
+
+    for solver in eigen_solver:
+        transformed_X = np.zeros((20, 2))
+        for i in range(20):
+            kpca = KernelPCA(n_components=2, eigen_solver=solver, random_state=rng)
+            transformed_X[i, :] = kpca.fit_transform(X)[0]
+        assert_allclose(transformed_X, np.tile(transformed_X[0, :], 20).reshape(20, 2))
+
+
+@pytest.mark.parametrize("csr_container", CSR_CONTAINERS)
+def test_kernel_pca_sparse(csr_container, global_random_seed):
+    """Test that kPCA works on a sparse data input.
+
+    Same test as ``test_kernel_pca except inverse_transform`` since it's not
+    implemented for sparse matrices.
+    """
+    rng = np.random.RandomState(global_random_seed)
+    X_fit = csr_container(rng.random_sample((5, 4)))
+    X_pred = csr_container(rng.random_sample((2, 4)))
+
+    for eigen_solver in ("auto", "arpack", "randomized"):
+        for kernel in ("linear", "rbf", "poly"):
+            # transform fit data
+            kpca = KernelPCA(
+                4,
+                kernel=kernel,
+                eigen_solver=eigen_solver,
+                fit_inverse_transform=False,
+                random_state=0,
+            )
+            X_fit_transformed = kpca.fit_transform(X_fit)
+            X_fit_transformed2 = kpca.fit(X_fit).transform(X_fit)
+            assert_array_almost_equal(
+                np.abs(X_fit_transformed), np.abs(X_fit_transformed2)
+            )
+
+            # transform new data
+            X_pred_transformed = kpca.transform(X_pred)
+            assert X_pred_transformed.shape[1] == X_fit_transformed.shape[1]
+
+            # inverse transform: not available for sparse matrices
+            # XXX: should we raise another exception type here? For instance:
+            # NotImplementedError.
+            with pytest.raises(NotFittedError):
+                kpca.inverse_transform(X_pred_transformed)
+
+
+@pytest.mark.parametrize("solver", ["auto", "dense", "arpack", "randomized"])
+@pytest.mark.parametrize("n_features", [4, 10])
+def test_kernel_pca_linear_kernel(solver, n_features, global_random_seed):
+    """Test that kPCA with linear kernel is equivalent to PCA for all solvers.
+
+    KernelPCA with linear kernel should produce the same output as PCA.
+    """
+    rng = np.random.RandomState(global_random_seed)
+    X_fit = rng.random_sample((5, n_features))
+    X_pred = rng.random_sample((2, n_features))
+
+    # for a linear kernel, kernel PCA should find the same projection as PCA
+    # modulo the sign (direction)
+    # fit only the first four components: fifth is near zero eigenvalue, so
+    # can be trimmed due to roundoff error
+    n_comps = 3 if solver == "arpack" else 4
+    assert_array_almost_equal(
+        np.abs(KernelPCA(n_comps, eigen_solver=solver).fit(X_fit).transform(X_pred)),
+        np.abs(
+            PCA(n_comps, svd_solver=solver if solver != "dense" else "full")
+            .fit(X_fit)
+            .transform(X_pred)
+        ),
+    )
+
+
+def test_kernel_pca_n_components():
+    """Test that `n_components` is correctly taken into account for projections
+
+    For all solvers this tests that the output has the correct shape depending
+    on the selected number of components.
+    """
+    rng = np.random.RandomState(0)
+    X_fit = rng.random_sample((5, 4))
+    X_pred = rng.random_sample((2, 4))
+
+    for eigen_solver in ("dense", "arpack", "randomized"):
+        for c in [1, 2, 4]:
+            kpca = KernelPCA(n_components=c, eigen_solver=eigen_solver)
+            shape = kpca.fit(X_fit).transform(X_pred).shape
+
+            assert shape == (2, c)
+
+
+def test_remove_zero_eig():
+    """Check that the ``remove_zero_eig`` parameter works correctly.
+
+    Tests that the null-space (Zero) eigenvalues are removed when
+    remove_zero_eig=True, whereas they are not by default.
+    """
+    X = np.array([[1 - 1e-30, 1], [1, 1], [1, 1 - 1e-20]])
+
+    # n_components=None (default) => remove_zero_eig is True
+    kpca = KernelPCA()
+    Xt = kpca.fit_transform(X)
+    assert Xt.shape == (3, 0)
+
+    kpca = KernelPCA(n_components=2)
+    Xt = kpca.fit_transform(X)
+    assert Xt.shape == (3, 2)
+
+    kpca = KernelPCA(n_components=2, remove_zero_eig=True)
+    Xt = kpca.fit_transform(X)
+    assert Xt.shape == (3, 0)
+
+
+def test_leave_zero_eig():
+    """Non-regression test for issue #12141 (PR #12143)
+
+    This test checks that fit().transform() returns the same result as
+    fit_transform() in case of non-removed zero eigenvalue.
+    """
+    X_fit = np.array([[1, 1], [0, 0]])
+
+    # Assert that even with all np warnings on, there is no div by zero warning
+    with warnings.catch_warnings():
+        # There might be warnings about the kernel being badly conditioned,
+        # but there should not be warnings about division by zero.
+        # (Numpy division by zero warning can have many message variants, but
+        # at least we know that it is a RuntimeWarning so let's check only this)
+        warnings.simplefilter("error", RuntimeWarning)
+        with np.errstate(all="warn"):
+            k = KernelPCA(n_components=2, remove_zero_eig=False, eigen_solver="dense")
+            # Fit, then transform
+            A = k.fit(X_fit).transform(X_fit)
+            # Do both at once
+            B = k.fit_transform(X_fit)
+            # Compare
+            assert_array_almost_equal(np.abs(A), np.abs(B))
+
+
+def test_kernel_pca_precomputed(global_random_seed):
+    """Test that kPCA works with a precomputed kernel, for all solvers"""
+    rng = np.random.RandomState(global_random_seed)
+    X_fit = rng.random_sample((5, 4))
+    X_pred = rng.random_sample((2, 4))
+
+    for eigen_solver in ("dense", "arpack", "randomized"):
+        X_kpca = (
+            KernelPCA(4, eigen_solver=eigen_solver, random_state=0)
+            .fit(X_fit)
+            .transform(X_pred)
+        )
+
+        X_kpca2 = (
+            KernelPCA(
+                4, eigen_solver=eigen_solver, kernel="precomputed", random_state=0
+            )
+            .fit(np.dot(X_fit, X_fit.T))
+            .transform(np.dot(X_pred, X_fit.T))
+        )
+
+        X_kpca_train = KernelPCA(
+            4, eigen_solver=eigen_solver, kernel="precomputed", random_state=0
+        ).fit_transform(np.dot(X_fit, X_fit.T))
+
+        X_kpca_train2 = (
+            KernelPCA(
+                4, eigen_solver=eigen_solver, kernel="precomputed", random_state=0
+            )
+            .fit(np.dot(X_fit, X_fit.T))
+            .transform(np.dot(X_fit, X_fit.T))
+        )
+
+        assert_array_almost_equal(np.abs(X_kpca), np.abs(X_kpca2))
+
+        assert_array_almost_equal(np.abs(X_kpca_train), np.abs(X_kpca_train2))
+
+
+@pytest.mark.parametrize("solver", ["auto", "dense", "arpack", "randomized"])
+def test_kernel_pca_precomputed_non_symmetric(solver):
+    """Check that the kernel centerer works.
+
+    Tests that a non symmetric precomputed kernel is actually accepted
+    because the kernel centerer does its job correctly.
+    """
+
+    # a non symmetric gram matrix
+    K = [[1, 2], [3, 40]]
+    kpca = KernelPCA(
+        kernel="precomputed", eigen_solver=solver, n_components=1, random_state=0
+    )
+    kpca.fit(K)  # no error
+
+    # same test with centered kernel
+    Kc = [[9, -9], [-9, 9]]
+    kpca_c = KernelPCA(
+        kernel="precomputed", eigen_solver=solver, n_components=1, random_state=0
+    )
+    kpca_c.fit(Kc)
+
+    # comparison between the non-centered and centered versions
+    assert_array_equal(kpca.eigenvectors_, kpca_c.eigenvectors_)
+    assert_array_equal(kpca.eigenvalues_, kpca_c.eigenvalues_)
+
+
+def test_gridsearch_pipeline():
+    """Check that kPCA works as expected in a grid search pipeline
+
+    Test if we can do a grid-search to find parameters to separate
+    circles with a perceptron model.
+    """
+    X, y = make_circles(n_samples=400, factor=0.3, noise=0.05, random_state=0)
+    kpca = KernelPCA(kernel="rbf", n_components=2)
+    pipeline = Pipeline([("kernel_pca", kpca), ("Perceptron", Perceptron(max_iter=5))])
+    param_grid = dict(kernel_pca__gamma=2.0 ** np.arange(-2, 2))
+    grid_search = GridSearchCV(pipeline, cv=3, param_grid=param_grid)
+    grid_search.fit(X, y)
+    assert grid_search.best_score_ == 1
+
+
+def test_gridsearch_pipeline_precomputed():
+    """Check that kPCA works as expected in a grid search pipeline (2)
+
+    Test if we can do a grid-search to find parameters to separate
+    circles with a perceptron model. This test uses a precomputed kernel.
+    """
+    X, y = make_circles(n_samples=400, factor=0.3, noise=0.05, random_state=0)
+    kpca = KernelPCA(kernel="precomputed", n_components=2)
+    pipeline = Pipeline([("kernel_pca", kpca), ("Perceptron", Perceptron(max_iter=5))])
+    param_grid = dict(Perceptron__max_iter=np.arange(1, 5))
+    grid_search = GridSearchCV(pipeline, cv=3, param_grid=param_grid)
+    X_kernel = rbf_kernel(X, gamma=2.0)
+    grid_search.fit(X_kernel, y)
+    assert grid_search.best_score_ == 1
+
+
+def test_nested_circles():
+    """Check that kPCA projects in a space where nested circles are separable
+
+    Tests that 2D nested circles become separable with a perceptron when
+    projected in the first 2 kPCA using an RBF kernel, while raw samples
+    are not directly separable in the original space.
+    """
+    X, y = make_circles(n_samples=400, factor=0.3, noise=0.05, random_state=0)
+
+    # 2D nested circles are not linearly separable
+    train_score = Perceptron(max_iter=5).fit(X, y).score(X, y)
+    assert train_score < 0.8
+
+    # Project the circles data into the first 2 components of a RBF Kernel
+    # PCA model.
+    # Note that the gamma value is data dependent. If this test breaks
+    # and the gamma value has to be updated, the Kernel PCA example will
+    # have to be updated too.
+    kpca = KernelPCA(
+        kernel="rbf", n_components=2, fit_inverse_transform=True, gamma=2.0
+    )
+    X_kpca = kpca.fit_transform(X)
+
+    # The data is perfectly linearly separable in that space
+    train_score = Perceptron(max_iter=5).fit(X_kpca, y).score(X_kpca, y)
+    assert train_score == 1.0
+
+
+def test_kernel_conditioning():
+    """Check that ``_check_psd_eigenvalues`` is correctly called in kPCA
+
+    Non-regression test for issue #12140 (PR #12145).
+    """
+
+    # create a pathological X leading to small non-zero eigenvalue
+    X = [[5, 1], [5 + 1e-8, 1e-8], [5 + 1e-8, 0]]
+    kpca = KernelPCA(kernel="linear", n_components=2, fit_inverse_transform=True)
+    kpca.fit(X)
+
+    # check that the small non-zero eigenvalue was correctly set to zero
+    assert kpca.eigenvalues_.min() == 0
+    assert np.all(kpca.eigenvalues_ == _check_psd_eigenvalues(kpca.eigenvalues_))
+
+
+@pytest.mark.parametrize("solver", ["auto", "dense", "arpack", "randomized"])
+def test_precomputed_kernel_not_psd(solver):
+    """Check how KernelPCA works with non-PSD kernels depending on n_components
+
+    Tests for all methods what happens with a non PSD gram matrix (this
+    can happen in an isomap scenario, or with custom kernel functions, or
+    maybe with ill-posed datasets).
+
+    When ``n_component`` is large enough to capture a negative eigenvalue, an
+    error should be raised. Otherwise, KernelPCA should run without error
+    since the negative eigenvalues are not selected.
+    """
+
+    # a non PSD kernel with large eigenvalues, already centered
+    # it was captured from an isomap call and multiplied by 100 for compacity
+    K = [
+        [4.48, -1.0, 8.07, 2.33, 2.33, 2.33, -5.76, -12.78],
+        [-1.0, -6.48, 4.5, -1.24, -1.24, -1.24, -0.81, 7.49],
+        [8.07, 4.5, 15.48, 2.09, 2.09, 2.09, -11.1, -23.23],
+        [2.33, -1.24, 2.09, 4.0, -3.65, -3.65, 1.02, -0.9],
+        [2.33, -1.24, 2.09, -3.65, 4.0, -3.65, 1.02, -0.9],
+        [2.33, -1.24, 2.09, -3.65, -3.65, 4.0, 1.02, -0.9],
+        [-5.76, -0.81, -11.1, 1.02, 1.02, 1.02, 4.86, 9.75],
+        [-12.78, 7.49, -23.23, -0.9, -0.9, -0.9, 9.75, 21.46],
+    ]
+    # this gram matrix has 5 positive eigenvalues and 3 negative ones
+    # [ 52.72,   7.65,   7.65,   5.02,   0.  ,  -0.  ,  -6.13, -15.11]
+
+    # 1. ask for enough components to get a significant negative one
+    kpca = KernelPCA(kernel="precomputed", eigen_solver=solver, n_components=7)
+    # make sure that the appropriate error is raised
+    with pytest.raises(ValueError, match="There are significant negative eigenvalues"):
+        kpca.fit(K)
+
+    # 2. ask for a small enough n_components to get only positive ones
+    kpca = KernelPCA(kernel="precomputed", eigen_solver=solver, n_components=2)
+    if solver == "randomized":
+        # the randomized method is still inconsistent with the others on this
+        # since it selects the eigenvalues based on the largest 2 modules, not
+        # on the largest 2 values.
+        #
+        # At least we can ensure that we return an error instead of returning
+        # the wrong eigenvalues
+        with pytest.raises(
+            ValueError, match="There are significant negative eigenvalues"
+        ):
+            kpca.fit(K)
+    else:
+        # general case: make sure that it works
+        kpca.fit(K)
+
+
+@pytest.mark.parametrize("n_components", [4, 10, 20])
+def test_kernel_pca_solvers_equivalence(n_components):
+    """Check that 'dense' 'arpack' & 'randomized' solvers give similar results"""
+
+    # Generate random data
+    n_train, n_test = 1_000, 100
+    X, _ = make_circles(
+        n_samples=(n_train + n_test), factor=0.3, noise=0.05, random_state=0
+    )
+    X_fit, X_pred = X[:n_train, :], X[n_train:, :]
+
+    # reference (full)
+    ref_pred = (
+        KernelPCA(n_components, eigen_solver="dense", random_state=0)
+        .fit(X_fit)
+        .transform(X_pred)
+    )
+
+    # arpack
+    a_pred = (
+        KernelPCA(n_components, eigen_solver="arpack", random_state=0)
+        .fit(X_fit)
+        .transform(X_pred)
+    )
+    # check that the result is still correct despite the approx
+    assert_array_almost_equal(np.abs(a_pred), np.abs(ref_pred))
+
+    # randomized
+    r_pred = (
+        KernelPCA(n_components, eigen_solver="randomized", random_state=0)
+        .fit(X_fit)
+        .transform(X_pred)
+    )
+    # check that the result is still correct despite the approximation
+    assert_array_almost_equal(np.abs(r_pred), np.abs(ref_pred))
+
+
+def test_kernel_pca_inverse_transform_reconstruction():
+    """Test if the reconstruction is a good approximation.
+
+    Note that in general it is not possible to get an arbitrarily good
+    reconstruction because of kernel centering that does not
+    preserve all the information of the original data.
+    """
+    X, *_ = make_blobs(n_samples=100, n_features=4, random_state=0)
+
+    kpca = KernelPCA(
+        n_components=20, kernel="rbf", fit_inverse_transform=True, alpha=1e-3
+    )
+    X_trans = kpca.fit_transform(X)
+    X_reconst = kpca.inverse_transform(X_trans)
+    assert np.linalg.norm(X - X_reconst) / np.linalg.norm(X) < 1e-1
+
+
+def test_kernel_pca_raise_not_fitted_error():
+    X = np.random.randn(15).reshape(5, 3)
+    kpca = KernelPCA()
+    kpca.fit(X)
+    with pytest.raises(NotFittedError):
+        kpca.inverse_transform(X)
+
+
+def test_32_64_decomposition_shape():
+    """Test that the decomposition is similar for 32 and 64 bits data
+
+    Non regression test for
+    https://github.com/scikit-learn/scikit-learn/issues/18146
+    """
+    X, y = make_blobs(
+        n_samples=30, centers=[[0, 0, 0], [1, 1, 1]], random_state=0, cluster_std=0.1
+    )
+    X = StandardScaler().fit_transform(X)
+    X -= X.min()
+
+    # Compare the shapes (corresponds to the number of non-zero eigenvalues)
+    kpca = KernelPCA()
+    assert kpca.fit_transform(X).shape == kpca.fit_transform(X.astype(np.float32)).shape
+
+
+def test_kernel_pca_feature_names_out():
+    """Check feature names out for KernelPCA."""
+    X, *_ = make_blobs(n_samples=100, n_features=4, random_state=0)
+    kpca = KernelPCA(n_components=2).fit(X)
+
+    names = kpca.get_feature_names_out()
+    assert_array_equal([f"kernelpca{i}" for i in range(2)], names)
+
+
+def test_kernel_pca_inverse_correct_gamma(global_random_seed):
+    """Check that gamma is set correctly when not provided.
+
+    Non-regression test for #26280
+    """
+    rng = np.random.RandomState(global_random_seed)
+    X = rng.random_sample((5, 4))
+
+    kwargs = {
+        "n_components": 2,
+        "random_state": rng,
+        "fit_inverse_transform": True,
+        "kernel": "rbf",
+    }
+
+    expected_gamma = 1 / X.shape[1]
+    kpca1 = KernelPCA(gamma=None, **kwargs).fit(X)
+    kpca2 = KernelPCA(gamma=expected_gamma, **kwargs).fit(X)
+
+    assert kpca1.gamma_ == expected_gamma
+    assert kpca2.gamma_ == expected_gamma
+
+    X1_recon = kpca1.inverse_transform(kpca1.transform(X))
+    X2_recon = kpca2.inverse_transform(kpca1.transform(X))
+
+    assert_allclose(X1_recon, X2_recon)
+
+
+def test_kernel_pca_pandas_output():
+    """Check that KernelPCA works with pandas output when the solver is arpack.
+
+    Non-regression test for:
+    https://github.com/scikit-learn/scikit-learn/issues/27579
+    """
+    pytest.importorskip("pandas")
+    X, _ = load_iris(as_frame=True, return_X_y=True)
+    with sklearn.config_context(transform_output="pandas"):
+        KernelPCA(n_components=2, eigen_solver="arpack").fit_transform(X)