Merge pull request matplotlib#29125 from timhoffm/colormap-monochrome

Make ListedColormap.monochrome a property

Author: timhoffm

lib/matplotlib/colors.py

@@ -1191,17 +1191,13 @@ class ListedColormap(Colormap):
         the list will be extended by repetition.
     """
     def __init__(self, colors, name='from_list', N=None):
-        self.monochrome = False  # Are all colors identical? (for contour.py)
         if N is None:
             self.colors = colors
             N = len(colors)
         else:
             if isinstance(colors, str):
                 self.colors = [colors] * N
-                self.monochrome = True
             elif np.iterable(colors):
-                if len(colors) == 1:
-                    self.monochrome = True
                 self.colors = list(
                     itertools.islice(itertools.cycle(colors), N))
             else:
@@ -1211,7 +1207,6 @@ def __init__(self, colors, name='from_list', N=None):
                     pass
                 else:
                     self.colors = [gray] * N
-                self.monochrome = True
         super().__init__(name, N)
 
     def _init(self):
@@ -1220,6 +1215,21 @@ def _init(self):
         self._isinit = True
         self._set_extremes()
 
+    @property
+    def monochrome(self):
+        """Return whether all colors in the colormap are identical."""
+        # Replacement for the attribute *monochrome*. This ensures a consistent
+        # response independent of the way the ListedColormap was created, which
+        # was not the case for the manually set attribute.
+        #
+        # TODO: It's a separate discussion whether we need this property on
+        #       colormaps at all (at least as public API). It's a very special edge
+        #       case and we only use it for contours internally.
+        if not self._isinit:
+            self._init()
+
+        return self.N <= 1 or np.all(self._lut[0] == self._lut[1:self.N])
+
     def resampled(self, lutsize):
         """Return a new colormap with *lutsize* entries."""
         colors = self(np.linspace(0, 1, lutsize))

lib/matplotlib/colors.pyi

@@ -130,11 +130,12 @@ class LinearSegmentedColormap(Colormap):
     def reversed(self, name: str | None = ...) -> LinearSegmentedColormap: ...
 
 class ListedColormap(Colormap):
-    monochrome: bool
     colors: ArrayLike | ColorType
     def __init__(
         self, colors: ArrayLike | ColorType, name: str = ..., N: int | None = ...
     ) -> None: ...
+    @property
+    def monochrome(self) -> bool: ...
     def resampled(self, lutsize: int) -> ListedColormap: ...
     def reversed(self, name: str | None = ...) -> ListedColormap: ...
 

lib/matplotlib/tests/test_colors.py

@@ -74,6 +74,12 @@ def test_resampled():
     assert_array_almost_equal(lc(np.nan), lc3(np.nan))
 
 
+def test_monochrome():
+    assert mcolors.ListedColormap(["red"]).monochrome
+    assert mcolors.ListedColormap(["red"] * 5).monochrome
+    assert not mcolors.ListedColormap(["red", "green"]).monochrome
+
+
 def test_colormaps_get_cmap():
     cr = mpl.colormaps