From 5090b264dbc60899ecef0cd0e0e4252668bd0542 Mon Sep 17 00:00:00 2001
From: Ian Thomas <ianthomas23@gmail.com>
Date: Fri, 14 Mar 2025 08:38:45 +0000
Subject: [PATCH] Backport PR #29721: FIX: pyplot auto-backend detection
 case-sensitivity fixup

---
 lib/matplotlib/pyplot.py              | 15 +++++++++------
 lib/matplotlib/tests/test_rcparams.py | 25 ++++++++++++++++++++++++-
 2 files changed, 33 insertions(+), 7 deletions(-)

diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
index 48aea1b3bd9c..83b380505d5b 100644
--- a/lib/matplotlib/pyplot.py
+++ b/lib/matplotlib/pyplot.py
@@ -2715,12 +2715,15 @@ def polar(*args, **kwargs) -> list[Line2D]:
 # If rcParams['backend_fallback'] is true, and an interactive backend is
 # requested, ignore rcParams['backend'] and force selection of a backend that
 # is compatible with the current running interactive framework.
-if (rcParams["backend_fallback"]
-        and rcParams._get_backend_or_none() in (  # type: ignore[attr-defined]
-            set(backend_registry.list_builtin(BackendFilter.INTERACTIVE)) -
-            {'webagg', 'nbagg'})
-        and cbook._get_running_interactive_framework()):
-    rcParams._set("backend", rcsetup._auto_backend_sentinel)
+if rcParams["backend_fallback"]:
+    requested_backend = rcParams._get_backend_or_none()  # type: ignore[attr-defined]
+    requested_backend = None if requested_backend is None else requested_backend.lower()
+    available_backends = backend_registry.list_builtin(BackendFilter.INTERACTIVE)
+    if (
+        requested_backend in (set(available_backends) - {'webagg', 'nbagg'})
+        and cbook._get_running_interactive_framework()
+    ):
+        rcParams._set("backend", rcsetup._auto_backend_sentinel)
 
 # fmt: on
 
diff --git a/lib/matplotlib/tests/test_rcparams.py b/lib/matplotlib/tests/test_rcparams.py
index 13633956c349..bea5e90ea4e5 100644
--- a/lib/matplotlib/tests/test_rcparams.py
+++ b/lib/matplotlib/tests/test_rcparams.py
@@ -521,10 +521,11 @@ def test_rcparams_reset_after_fail():
 
 
 @pytest.mark.skipif(sys.platform != "linux", reason="Linux only")
-def test_backend_fallback_headless(tmp_path):
+def test_backend_fallback_headless_invalid_backend(tmp_path):
     env = {**os.environ,
            "DISPLAY": "", "WAYLAND_DISPLAY": "",
            "MPLBACKEND": "", "MPLCONFIGDIR": str(tmp_path)}
+    # plotting should fail with the tkagg backend selected in a headless environment
     with pytest.raises(subprocess.CalledProcessError):
         subprocess_run_for_testing(
             [sys.executable, "-c",
@@ -536,6 +537,28 @@ def test_backend_fallback_headless(tmp_path):
             env=env, check=True, stderr=subprocess.DEVNULL)
 
 
+@pytest.mark.skipif(sys.platform != "linux", reason="Linux only")
+def test_backend_fallback_headless_auto_backend(tmp_path):
+    # specify a headless mpl environment, but request a graphical (tk) backend
+    env = {**os.environ,
+           "DISPLAY": "", "WAYLAND_DISPLAY": "",
+           "MPLBACKEND": "TkAgg", "MPLCONFIGDIR": str(tmp_path)}
+
+    # allow fallback to an available interactive backend explicitly in configuration
+    rc_path = tmp_path / "matplotlibrc"
+    rc_path.write_text("backend_fallback: true")
+
+    # plotting should succeed, by falling back to use the generic agg backend
+    backend = subprocess_run_for_testing(
+        [sys.executable, "-c",
+         "import matplotlib.pyplot;"
+         "matplotlib.pyplot.plot(42);"
+         "print(matplotlib.get_backend());"
+         ],
+        env=env, text=True, check=True, capture_output=True).stdout
+    assert backend.strip().lower() == "agg"
+
+
 @pytest.mark.skipif(
     sys.platform == "linux" and not _c_internal_utils.xdisplay_is_valid(),
     reason="headless")