Fix boolean contour defaults (swev-id: matplotlib__matplotlib-24870)

doc/users/next_whats_new/contour_boolean_levels.rst

@@ -0,0 +1,9 @@
+Boolean contour defaults
+------------------------
+
+`.Axes.contour` and `.Axes.contourf` now recognise boolean input arrays when
+``levels`` is omitted. Line contours default to a single threshold at ``0.5``
+while filled contours draw the three regions ``[0.0, 0.5, 1.0]`` to
+distinguish ``False`` and ``True`` values. Passing explicit ``levels``, using a
+custom locator, or enabling logarithmic scaling continues to behave exactly as
+before.

lib/matplotlib/contour.py

@@ -1123,15 +1123,24 @@ def _process_contour_level_args(self, args):
         """
         if self.levels is None:
             if len(args) == 0:
-                levels_arg = 7  # Default, hard-wired.
+                levels_arg = None
             else:
                 levels_arg = args[0]
         else:
             levels_arg = self.levels
-        if isinstance(levels_arg, Integral):
-            self.levels = self._autolev(levels_arg)
+        if (levels_arg is None and getattr(self, "_is_bool_z", False)
+                and self.locator is None and not self.logscale):
+            if self.filled:
+                self.levels = np.array([0.0, 0.5, 1.0], dtype=np.float64)
+            else:
+                self.levels = np.array([0.5], dtype=np.float64)
         else:
-            self.levels = np.asarray(levels_arg, np.float64)
+            if levels_arg is None:
+                levels_arg = 7  # Default, hard-wired.
+            if isinstance(levels_arg, Integral):
+                self.levels = self._autolev(levels_arg)
+            else:
+                self.levels = np.asarray(levels_arg, np.float64)
 
         if not self.filled:
             inside = (self.levels > self.zmin) & (self.levels < self.zmax)
@@ -1446,8 +1455,11 @@ def _contour_args(self, args, kwargs):
         else:
             fn = 'contour'
         nargs = len(args)
+        self._is_bool_z = False
         if nargs <= 2:
-            z = ma.asarray(args[0], dtype=np.float64)
+            z_input = args[0]
+            self._is_bool_z = ma.getdata(ma.asarray(z_input)).dtype.kind == "b"
+            z = ma.asarray(z_input, dtype=np.float64)
             x, y = self._initialize_x_y(z)
             args = args[1:]
         elif nargs <= 4:
@@ -1475,7 +1487,9 @@ def _check_xyz(self, args, kwargs):
 
         x = np.asarray(x, dtype=np.float64)
         y = np.asarray(y, dtype=np.float64)
-        z = ma.asarray(args[2], dtype=np.float64)
+        z_input = args[2]
+        self._is_bool_z = ma.getdata(ma.asarray(z_input)).dtype.kind == "b"
+        z = ma.asarray(z_input, dtype=np.float64)
 
         if z.ndim != 2:
             raise TypeError(f"Input z must be 2D, not {z.ndim}D")
@@ -1585,6 +1599,14 @@ def _initialize_x_y(self, z):
     If array-like, draw contour lines at the specified levels.
     The values must be in increasing order.
 
+    .. note::
+
+        If *Z* has a boolean dtype and *levels* is omitted (or ``None``),
+        `.contour` adds a single level at ``0.5`` and `.contourf` draws
+        filled regions using the levels ``[0.0, 0.5, 1.0]``. Supplying
+        explicit *levels*, configuring *locator*, or using a logarithmic
+        scale preserves the provided behaviour.
+
 Returns
 -------
 `~.contour.QuadContourSet`

lib/matplotlib/tests/test_contour.py

@@ -84,6 +84,57 @@ def test_contour_Nlevels():
     assert (cs1.levels == cs2.levels).all()
 
 
+def test_contour_bool_default_levels():
+    z = np.array([[True, False], [False, True]])
+
+    fig, ax = plt.subplots()
+    cs = ax.contour(z)
+    assert np.allclose(cs.levels, [0.5])
+
+
+def test_contourf_bool_default_levels():
+    z = np.array([[True, False], [False, True]])
+
+    fig, ax = plt.subplots()
+    cs = ax.contourf(z)
+    assert np.allclose(cs.levels, [0.0, 0.5, 1.0])
+
+
+def test_contour_bool_explicit_levels_preserved():
+    z = np.array([[True, False], [False, True]])
+
+    fig, ax = plt.subplots()
+    levels = [0.25, 0.75]
+    cs = ax.contour(z, levels=levels)
+    assert np.allclose(cs.levels, levels)
+
+
+def test_contourf_masked_bool_default_levels():
+    data = np.array([[True, False], [False, True]])
+    mask = [[False, True], [False, False]]
+    z = np.ma.array(data, mask=mask)
+
+    fig, ax = plt.subplots()
+    cs = ax.contourf(z)
+    assert np.allclose(cs.levels, [0.0, 0.5, 1.0])
+
+
+def test_contour_int_data_no_bool_special_case():
+    z = np.array([[1, 0], [0, 1]], dtype=int)
+
+    fig, ax = plt.subplots()
+    cs = ax.contour(z)
+    assert not np.allclose(cs.levels, [0.5])
+
+
+def test_contourf_bool_lognorm_no_override():
+    z = np.ones((2, 2), dtype=bool)
+
+    fig, ax = plt.subplots()
+    cs = ax.contourf(z, norm=LogNorm())
+    assert not np.allclose(cs.levels, [0.0, 0.5, 1.0])
+
+
 def test_contour_badlevel_fmt():
     # Test edge case from https://github.com/matplotlib/matplotlib/issues/9742
     # User supplied fmt for each level as a dictionary, but Matplotlib changed