Make ListedColormap.monochrome a property

The calculated property replaces the attribute *monochome*, which was
manually set on `__init__`, but was not correctly set for all possible
inputs.

This property ensures consistency and simplifies initialization at the
cost of some computation overhead to determine whether the colormap is
monochrome.

The computation cost is bearable (even without caching), because it's
only used in `ContourSet._process_colors`.

It's a separate discussion whether we need this property on colormaps at
all (at least as public API). Usually, colormaps are not monochrome
and monochrome colormaps are a very special edge case used in contours
only. We may eventually deprecate it, but since it is currently public
API, let's leave it for now.

There's also a technical API incompatibility in that users cannot set
the attribute anymore, but I'd argue that that has never been intended
and there's no practical use-case, so I refrain from the extra hassle
of allowing setting this property.

Co-authored-by: Greg Lucas <greg.m.lucas@gmail.com>

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