Fix contour behavior when Z is constant (zmin == zmax)

m-sahare · m-sahare · commit de3e7da94814 · 2026-04-20T15:14:45.000+05:30
diff --git a/lib/matplotlib/contour.py b/lib/matplotlib/contour.py
@@ -6,7 +6,7 @@
 import functools
 import math
 from numbers import Integral
-
+import warnings
 import numpy as np
 from numpy import ma
 
@@ -998,6 +998,15 @@ def _autolev(self):
         one contour line, but two filled regions, and therefore
         three levels to provide boundaries for both regions.
         """
+       
+
+        if self.zmin == self.zmax:
+            warnings.warn(
+                "Z is constant; contour levels are not meaningful.",
+                stacklevel=2
+            )
+            return np.array([self.zmin])
+
         lev = self.locator.tick_values(self.zmin, self.zmax)
 
         try:
@@ -1006,7 +1015,7 @@ def _autolev(self):
         except AttributeError:
             pass
 
-        # Trim excess levels the locator may have supplied.
+    # Trim excess levels the locator may have supplied.
         under = np.nonzero(lev < self.zmin)[0]
         i0 = under[-1] if len(under) else 0
         over = np.nonzero(lev > self.zmax)[0]
diff --git a/lib/matplotlib/tests/test_contour.py b/lib/matplotlib/tests/test_contour.py
@@ -876,3 +876,16 @@ def test_contour_aliases(fig_test, fig_ref):
 def test_contour_singular_color():
     with pytest.raises(TypeError):
         plt.figure().add_subplot().contour([[0, 1], [2, 3]], color="r")
+
+def test_contour_constant_z_warns_and_single_level():
+    Z = np.ones((10, 10))
+
+    fig, ax = plt.subplots()
+
+    # Expect a warning
+    with pytest.warns(UserWarning, match="Z is constant"):
+        cs = ax.contour(Z)
+
+    # Ensure only one level is created
+    assert len(cs.levels) == 1
+    assert cs.levels[0] == 1.0
