code: def test_peak_finder():
      """Test the peak detection method."""
      # check for random data
      rng = np.random.RandomState(42)
      peak_inds, peak_mags = peak_finder(rng.randn(20))
  
      assert_equal(peak_inds.dtype, np.dtype("int64"))
      assert_equal(peak_mags.dtype, np.dtype("float64"))
  
      # check for empty array as created in the #5025
      with pytest.raises(ValueError):
          peak_finder(np.arange(2, 1, 0.05))
  
      # check for empty array
      with pytest.raises(ValueError):
          peak_finder([])
  
      # check for monotonic function
      peak_inds, peak_mags = peak_finder(np.arange(1, 2, 0.05))
  
      assert_equal(peak_inds.dtype, np.dtype("int64"))
      assert_equal(peak_mags.dtype, np.dtype("float64"))
  
      # check for no peaks
      peak_inds, peak_mags = peak_finder(np.zeros(20))
  
      assert_equal(peak_inds.dtype, np.dtype("int64"))
      assert_equal(peak_mags.dtype, np.dtype("float64"))
  
      # check values
      peak_inds, peak_mags = peak_finder([0, 2, 5, 0, 6, -1])
      assert_array_equal(peak_inds, [2, 4])
  
imports: import numpy
  import pytest
  from numpy.testing import assert_array_equal, assert_equal
  from mne.preprocessing import peak_finder
label_desc: Use image node_id 1 for calling a global function with example usage: test_peak_finder() without return types