I am done

rl/Lib/site-packages/matplotlib/tests/test_backend_webagg.py

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+import os
+import sys
+import pytest
+
+import matplotlib.backends.backend_webagg_core
+from matplotlib.testing import subprocess_run_for_testing
+
+
+@pytest.mark.parametrize("backend", ["webagg", "nbagg"])
+def test_webagg_fallback(backend):
+    pytest.importorskip("tornado")
+    if backend == "nbagg":
+        pytest.importorskip("IPython")
+    env = dict(os.environ)
+    if sys.platform != "win32":
+        env["DISPLAY"] = ""
+
+    env["MPLBACKEND"] = backend
+
+    test_code = (
+        "import os;"
+        + f"assert os.environ['MPLBACKEND'] == '{backend}';"
+        + "import matplotlib.pyplot as plt; "
+        + "print(plt.get_backend());"
+        f"assert '{backend}' == plt.get_backend().lower();"
+    )
+    subprocess_run_for_testing([sys.executable, "-c", test_code], env=env, check=True)
+
+
+def test_webagg_core_no_toolbar():
+    fm = matplotlib.backends.backend_webagg_core.FigureManagerWebAgg
+    assert fm._toolbar2_class is None