code: def test_contourf_decreasing_levels():
      # github issue 5477.
      z = [[0.1, 0.3], [0.5, 0.7]]
      plt.figure()
      with pytest.raises(ValueError):
          plt.contourf(z, [1.0, 0.0])
  
imports: import datetime
  import platform
  import re
  from unittest import mock
  import contourpy
  import numpy
  from numpy.testing import assert_array_almost_equal, assert_array_almost_equal_nulp, assert_array_equal
  import matplotlib
  from matplotlib import pyplot, rc_context, ticker
  from matplotlib.colors import LogNorm, same_color
  import matplotlib.patches
  from matplotlib.testing.decorators import check_figures_equal, image_comparison
  import pytest
label_desc: Use image node_id 17 for calling a global function with example usage: test_contourf_decreasing_levels() without return types