code: 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
  
imports: import warnings
  import numpy
  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
label_desc: Use image node_id 14 for calling a global function with example usage: test_nested_circles() without return types