matplotlib
diff --git a/‎lib/matplotlib/artist.py‎
Lines changed: 21 additions & 0 deletions b/‎lib/matplotlib/artist.py‎
Lines changed: 21 additions & 0 deletions
diff --git a/‎lib/matplotlib/collections.py‎
Lines changed: 17 additions & 18 deletions b/‎lib/matplotlib/collections.py‎
Lines changed: 17 additions & 18 deletions
diff --git a/‎lib/matplotlib/colors.py‎
Lines changed: 1 addition & 0 deletions b/‎lib/matplotlib/colors.py‎
Lines changed: 1 addition & 0 deletions
@@ -958,6 +958,27 @@ def set_alpha(self, alpha):
         """
         if alpha is not None and not isinstance(alpha, Number):
             raise TypeError('alpha must be a float or None')
+        if alpha < 0 or alpha > 1:
+            raise ValueError(f'alpha of {alpha} is outside 0-1 range')
+        self._alpha = alpha
+        self.pchanged()
+        self.stale = True
+
+    def set_alpha_or_array(self, alpha):
+        """
+        Set the alpha value used for blending - not supported on all backends.
+
+        Parameters
+        ----------
+        alpha : array-like or float or None
+        """
+        if not np.iterable(alpha):
+            self.set_alpha(alpha)
+            return
+        alpha = np.asarray(alpha)
+        if alpha.min() < 0 or alpha.max() > 1:
+            raise ValueError(f'alpha must be between 0 and 1, inclusive, '
+                             'but min is {alpha.min()}, max is {alpha.max()}')
         self._alpha = alpha
         self.pchanged()
         self.stale = True
 
@@ -832,29 +832,15 @@ def set_edgecolor(self, c):
         self._set_edgecolor(c)
 
     def set_alpha(self, alpha):
-        """
-        Set the alpha tranparencies of the collection.  *alpha* must be
-        a float, a sequence of floats matching an array to be
-        colormapped, or *None*.
-
-        ACCEPTS: float or None
-        """
-        if alpha is not None:
-            if np.iterable(alpha):
-                alpha = np.asarray(alpha, dtype=float)
-            else:
-                try:
-                    alpha = float(alpha)
-                except TypeError:
-                    raise TypeError('alpha must be a float,'
-                                    ' sequence of float, or None')
-        super().set_alpha(alpha)
+        artist.Artist.set_alpha_or_array(self, alpha)
         self._update_dict['array'] = True
         if alpha is not None and isinstance(alpha, np.ndarray):
             return
         self._set_facecolor(self._original_facecolor)
         self._set_edgecolor(self._original_edgecolor)
 
+    set_alpha.__doc__ = artist.Artist.set_alpha_or_array.__doc__
+
     def get_linewidth(self):
         return self._linewidths
 
@@ -865,11 +851,24 @@ def update_scalarmappable(self):
         """Update colors from the scalar mappable array, if it is not None."""
         if self._A is None:
             return
-        # QuadMesh can map 2d arrays
+        # QuadMesh can map 2d arrays (but pcolormesh supplies 1d array)
         if self._A.ndim > 1 and not isinstance(self, QuadMesh):
             raise ValueError('Collections can only map rank 1 arrays')
         if not self._check_update("array"):
             return
+        if np.iterable(self._alpha) and isinstance(self, QuadMesh):
+            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}.')
+            # Handle the unfortunate behavior of pcolormesh in flattening
+            # its color array here rather than in to_rgba.
+            self._alpha = self._alpha.reshape(self._A.shape)
+
         if self._is_filled:
             self._facecolors = self.to_rgba(self._A, self._alpha)
         elif self._is_stroked:
 
@@ -609,6 +609,7 @@ def __call__(self, X, alpha=None, bytes=False):
             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:
                     rgba[mask_bad] = (0, 0, 0, 0)