From bf640547c0cd6a4b0fb7634e127430a30d9c5b49 Mon Sep 17 00:00:00 2001
From: Praful Gulani <59774145+prafulgulani@users.noreply.github.com>
Date: Fri, 28 Feb 2025 20:54:41 +0530
Subject: [PATCH] Backport PR #29590: Blocked set_clim() callbacks to prevent
 inconsistent state (#29522)

---
 lib/matplotlib/colorizer.py         | 21 ++++++++++++++++-----
 lib/matplotlib/tests/test_colors.py | 17 +++++++++++++++++
 2 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/lib/matplotlib/colorizer.py b/lib/matplotlib/colorizer.py
index 4aebe7d0f5dc..b4223f389804 100644
--- a/lib/matplotlib/colorizer.py
+++ b/lib/matplotlib/colorizer.py
@@ -261,16 +261,27 @@ def set_clim(self, vmin=None, vmax=None):
              .. ACCEPTS: (vmin: float, vmax: float)
         """
         # If the norm's limits are updated self.changed() will be called
-        # through the callbacks attached to the norm
+        # through the callbacks attached to the norm, this causes an inconsistent
+        # state, to prevent this blocked context manager is used
         if vmax is None:
             try:
                 vmin, vmax = vmin
             except (TypeError, ValueError):
                 pass
-        if vmin is not None:
-            self.norm.vmin = colors._sanitize_extrema(vmin)
-        if vmax is not None:
-            self.norm.vmax = colors._sanitize_extrema(vmax)
+
+        orig_vmin_vmax = self.norm.vmin, self.norm.vmax
+
+        # Blocked context manager prevents callbacks from being triggered
+        # until both vmin and vmax are updated
+        with self.norm.callbacks.blocked(signal='changed'):
+            if vmin is not None:
+                self.norm.vmin = colors._sanitize_extrema(vmin)
+            if vmax is not None:
+                self.norm.vmax = colors._sanitize_extrema(vmax)
+
+        # emit a update signal if the limits are changed
+        if orig_vmin_vmax != (self.norm.vmin, self.norm.vmax):
+            self.norm.callbacks.process('changed')
 
     def get_clim(self):
         """
diff --git a/lib/matplotlib/tests/test_colors.py b/lib/matplotlib/tests/test_colors.py
index cc6cb1bb11a7..1d13b940026b 100644
--- a/lib/matplotlib/tests/test_colors.py
+++ b/lib/matplotlib/tests/test_colors.py
@@ -1553,6 +1553,23 @@ def test_norm_deepcopy():
     assert norm2.vmin == norm.vmin
 
 
+def test_set_clim_emits_single_callback():
+    data = np.array([[1, 2], [3, 4]])
+    fig, ax = plt.subplots()
+    image = ax.imshow(data, cmap='viridis')
+
+    callback = unittest.mock.Mock()
+    image.norm.callbacks.connect('changed', callback)
+
+    callback.assert_not_called()
+
+    # Call set_clim() to update the limits
+    image.set_clim(1, 5)
+
+    # Assert that only one "changed" callback is sent after calling set_clim()
+    callback.assert_called_once()
+
+
 def test_norm_callback():
     increment = unittest.mock.Mock(return_value=None)