Backport PR #21034: Make rcParams["backend"] backend fallback check rcParams identity first.

timhoffm · meeseeksmachine · commit 068a3e367a01 · 2021-09-16T21:36:09.000Z
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -663,7 +663,9 @@ def __getitem__(self, key):
                 version, name=key, obj_type="rcparam", alternative=alt_key)
             return dict.__getitem__(self, alt_key) if alt_key else None
 
-        elif key == "backend":
+        # In theory, this should only ever be used after the global rcParams
+        # has been set up, but better be safe e.g. in presence of breakpoints.
+        elif key == "backend" and self is globals().get("rcParams"):
             val = dict.__getitem__(self, key)
             if val is rcsetup._auto_backend_sentinel:
                 from matplotlib import pyplot as plt
diff --git a/lib/matplotlib/tests/test_rcparams.py b/lib/matplotlib/tests/test_rcparams.py
@@ -500,8 +500,10 @@ def test_backend_fallback_headful(tmpdir):
     backend = subprocess.check_output(
         [sys.executable, "-c",
          "import matplotlib as mpl; "
-         "assert dict.__getitem__(mpl.rcParams, 'backend') == "
-         "mpl.rcsetup._auto_backend_sentinel; "
+         "sentinel = mpl.rcsetup._auto_backend_sentinel; "
+         # Check that access on another instance does not resolve the sentinel.
+         "assert mpl.RcParams({'backend': sentinel})['backend'] == sentinel; "
+         "assert dict.__getitem__(mpl.rcParams, 'backend') == sentinel; "
          "import matplotlib.pyplot; "
          "print(matplotlib.get_backend())"],
         env=env, universal_newlines=True)
