Fixups from review

Fixup and clarification in colors; more tests

Tweak whats_new

Author: efiring

doc/users/next_whats_new/alpha_array.rst

@@ -1,7 +1,7 @@
 Transparency (alpha) can be set as an array in collections
 ----------------------------------------------------------
 Previously, the alpha value controlling tranparency in collections could be
-specified only as a scalar which was applied to all elements in the collection.
+specified only as a scalar applied to all elements in the collection.
 For example, all the markers in a `~.Axes.scatter` plot, or all the
 quadrilaterals in a `~.Axes.pcolormesh` plot, would have the same alpha value.
 

lib/matplotlib/artist.py

@@ -982,7 +982,7 @@ def _set_alpha_for_array(self, alpha):
             Artist.set_alpha(self, alpha)
             return
         alpha = np.asarray(alpha)
-        if not (alpha.min() >= 0 and alpha.max() <= 1):
+        if not (0 <= alpha.min() and alpha.max() <= 1):
             raise ValueError('alpha must be between 0 and 1, inclusive, '
                              f'but min is {alpha.min()}, max is {alpha.max()}')
         self._alpha = alpha

lib/matplotlib/collections.py

@@ -837,9 +837,9 @@ def set_alpha(self, alpha):
 
         Parameters
         ----------
-        alpha: float or array of float
+        alpha: float or array of float or None
             If not None, *alpha* values must be between 0 and 1, inclusive.
-            If an array is provided, it's length must match the number of
+            If an array is provided, its length must match the number of
             elements in the collection.  Masked values and nans are not
             supported.
         """
@@ -867,13 +867,13 @@ def update_scalarmappable(self):
             return
         if np.iterable(self._alpha):
             if self._alpha.size != self._A.size:
-                # This can occur with the deprecated behavior of 'flat'
-                # pcolormesh shading.  If we bring the current change in
-                # before that deprecated behavior is removed, we need to
-                # add the explanation to the message below.
                 raise ValueError(f'Data array shape, {self._A.shape} '
                                  'is incompatible with alpha array shape, '
-                                 f'{self._alpha.shape}.')
+                                 f'{self._alpha.shape}. '
+                                 'This can occur with the deprecated '
+                                 'behavior of the "flat" shading option, '
+                                 'in which a row and/or column of the data '
+                                 'array is dropped.')
             # pcolormesh, scatter, maybe others flatten their _A
             self._alpha = self._alpha.reshape(self._A.shape)
 

lib/matplotlib/colors.py

@@ -615,20 +615,20 @@ def __call__(self, X, alpha=None, bytes=False):
         if alpha is not None:
             if np.iterable(alpha):
                 alpha = np.asarray(alpha)
-                if not (alpha.shape == xa.shape):
-                    raise ValueError("alpha is array-like but it's shape"
+                if alpha.shape != xa.shape:
+                    raise ValueError("alpha is array-like but its shape"
                                      " %s doesn't match that of X %s" %
                                      (alpha.shape, xa.shape))
-
             alpha = np.clip(alpha, 0, 1)
             if bytes:
                 alpha = (alpha * 255).astype(np.uint8)
             rgba[..., -1] = alpha
 
-            if (lut[-1] == 0).all() and mask_bad is not None:
-                if mask_bad.shape == xa.shape:
+            # If the "bad" color is all zeros, then ignore alpha input.
+            if (lut[-1] == 0).all() and np.any(mask_bad):
+                if np.iterable(mask_bad) and mask_bad.shape == xa.shape:
                     rgba[mask_bad] = (0, 0, 0, 0)
-                elif mask_bad:
+                else:
                     rgba[..., :] = (0, 0, 0, 0)
 
         if not np.iterable(X):

lib/matplotlib/image.py

@@ -274,7 +274,7 @@ def set_alpha(self, alpha):
 
         Parameters
         ----------
-        alpha : float or 2-d array or None
+        alpha : float or 2D array-like or None
         """
         martist.Artist._set_alpha_for_array(self, alpha)
         if np.ndim(alpha) not in (0, 2):

lib/matplotlib/tests/test_colors.py

@@ -1189,8 +1189,25 @@ def test_get_under_over_bad():
 
 def test_colormap_alpha_array():
     cmap = plt.get_cmap('viridis')
+    vals = [-1, 0.5, 2]  # under, valid, over
     with pytest.raises(ValueError, match="alpha is array-like but"):
-        cmap([1, 2, 3], alpha=[1, 1, 1, 1])
-    alpha = [0.1, 0.2, 0.3]
-    c = cmap([1, 2, 3], alpha=alpha)
+        cmap(vals, alpha=[1, 1, 1, 1])
+    alpha = np.array([0.1, 0.2, 0.3])
+    c = cmap(vals, alpha=alpha)
     assert_array_equal(c[:, -1], alpha)
+    c = cmap(vals, alpha=alpha, bytes=True)
+    assert_array_equal(c[:, -1], (alpha * 255).astype(np.uint8))
+
+
+def test_colormap_bad_data_with_alpha():
+    cmap = plt.get_cmap('viridis')
+    c = cmap(np.nan, alpha=0.5)
+    assert c == (0, 0, 0, 0)
+    c = cmap([0.5, np.nan], alpha=0.5)
+    assert_array_equal(c[1], (0, 0, 0, 0))
+    c = cmap([0.5, np.nan], alpha=[0.1, 0.2])
+    assert_array_equal(c[1], (0, 0, 0, 0))
+    c = cmap([[np.nan, 0.5], [0, 0]], alpha=0.5)
+    assert_array_equal(c[0, 0], (0, 0, 0, 0))
+    c = cmap([[np.nan, 0.5], [0, 0]], alpha=np.full((2, 2), 0.5))
+    assert_array_equal(c[0, 0], (0, 0, 0, 0))