code: def test_polar_to_cartesian():
      """Test helper transform function from polar to cartesian."""
      r = 1
      theta = np.pi
      # expected values are (-1, 0)
      x = r * np.cos(theta)
      y = r * np.sin(theta)
      coord = _pol_to_cart(np.array([[r, theta]]))[0]
      # np.pi is an approx since pi is irrational
      assert_allclose(coord, (x, y), atol=1e-7)
      assert_allclose(coord, (-1, 0), atol=1e-7)
      assert_allclose(coord, _polar_to_cartesian(theta, r), atol=1e-7)
      rng = np.random.RandomState(0)
      r = rng.randn(10)
      theta = rng.rand(10) * (2 * np.pi)
      polar = np.array((r, theta)).T
      assert_allclose(
          [_polar_to_cartesian(p[1], p[0]) for p in polar],
          _pol_to_cart(polar),
          atol=1e-7,
      )
  
imports: import itertools
  import os
  from pathlib import Path
  import numpy
  import pytest
  from numpy.testing import assert_allclose, assert_almost_equal, assert_array_equal, assert_array_less, assert_equal
  import mne
  from mne import read_trans, write_trans
  from mne.datasets import testing
  from mne.fixes import _get_img_fdata
  from mne.io import read_info
  from mne.transforms import _angle_between_quats, _average_quats, _cart_to_sph, _compute_r2, _euler_to_quat, _find_trans, _find_vector_rotation, _fit_matched_points, _get_trans, _MatchedDisplacementFieldInterpolator, _pol_to_cart, _quat_real, _quat_to_affine, _quat_to_euler, _read_fs_xfm, _sph_to_cart, _topo_to_sph, _validate_pipeline, _write_fs_xfm, apply_trans, combine_transforms, get_ras_to_neuromag_trans, invert_transform, quat_to_rot, rot_to_quat, rotation, rotation3d, rotation3d_align_z_axis, rotation_angles, translation
  from mne.transforms import _SphericalSurfaceWarp
label_desc: Use image node_id 9 for calling a global function with example usage: test_polar_to_cartesian() without return types