code: def test_backend_fallback_headless(tmp_path):
      env = {
          **os.environ,
          "DISPLAY": "",
          "WAYLAND_DISPLAY": "",
          "MPLBACKEND": "",
          "MPLCONFIGDIR": str(tmp_path),
      }
      with pytest.raises(subprocess.CalledProcessError):
          subprocess.run(
              [
                  sys.executable,
                  "-c",
                  "import matplotlib;"
                  "matplotlib.use('tkagg');"
                  "import matplotlib.pyplot;"
                  "matplotlib.pyplot.plot(42);",
              ],
              env=env,
              check=True,
              stderr=subprocess.DEVNULL,
          )
  
imports: import copy
  import os
  import subprocess
  import sys
  from unittest import mock
  from cycler import cycler, Cycler
  import pytest
  import matplotlib
  from matplotlib import _api, _c_internal_utils
  import matplotlib.pyplot
  import matplotlib.colors
  import numpy
  from matplotlib.rcsetup import validate_bool, validate_color, validate_colorlist, _validate_color_or_linecolor, validate_cycler, validate_float, validate_fontstretch, validate_fontweight, validate_hatch, validate_hist_bins, validate_int, validate_markevery, validate_stringlist, validate_sketch, _validate_linestyle, _listify_validator
label_desc: Use image node_id 21 for calling a global function with example usage: test_backend_fallback_headless(tmp_path) without return types