Merge pull request #15582 from story645/auto-backport-of-pr-15512-on-v3.1.x

timhoffm · web-flow · commit fccf3d9ec69b · 2019-11-01T08:41:24.000+01:00
Backport PR #15512 on branch v3.1.x
diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
@@ -2341,7 +2341,8 @@ def getname_val(identifier):
 # requested, ignore rcParams['backend'] and force selection of a backend that
 # is compatible with the current running interactive framework.
 if (rcParams["backend_fallback"]
-        and dict.__getitem__(rcParams, "backend") in _interactive_bk
+        and dict.__getitem__(rcParams, "backend") in (
+            set(_interactive_bk) - {'WebAgg', 'nbAgg'})
         and _get_running_interactive_framework()):
     dict.__setitem__(rcParams, "backend", rcsetup._auto_backend_sentinel)
 # Set up the backend.
diff --git a/lib/matplotlib/tests/test_backend_webagg.py b/lib/matplotlib/tests/test_backend_webagg.py
@@ -0,0 +1,28 @@
+import subprocess
+import os
+import sys
+import pytest
+
+
+@pytest.mark.parametrize("backend", ["webagg", "nbagg"])
+def test_webagg_fallback(backend):
+    if backend == "nbagg":
+        pytest.importorskip("IPython")
+    env = {}
+    if os.name == "nt":
+        env = dict(os.environ)
+    else:
+        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();"
+    )
+    ret = subprocess.call([sys.executable, "-c", test_code], env=env)
+
+    assert ret == 0
