Small cleanups to hatch.py.

- In Shapes.set_vertices_and_codes, handle `self.filled` just once at
  the beginning, and collect the vertices and codes parts in a list and
  concatenate them all at the end, rather than keeping track of a
  position in the buffers.
- Make SmallFilledCirecles inherit from Circles rather that
  SmallCircles, with which it shares no attribute.

Author: anntzer

doc/api/next_api_changes/behavior/20692-AL.rst

@@ -0,0 +1,3 @@
+``hatch.SmallFilledCircles`` now inherits from ``hatch.Circles``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+... rather than from ``hatch.SmallCircles``.

lib/matplotlib/hatch.py

@@ -101,28 +101,24 @@ def __init__(self, hatch, density):
     def set_vertices_and_codes(self, vertices, codes):
         offset = 1.0 / self.num_rows
         shape_vertices = self.shape_vertices * offset * self.size
-        if not self.filled:
-            inner_vertices = shape_vertices[::-1] * 0.9
         shape_codes = self.shape_codes
-        shape_size = len(shape_vertices)
-
-        cursor = 0
+        if not self.filled:
+            shape_vertices = np.concatenate(  # Forward, then backward.
+                [shape_vertices, shape_vertices[::-1] * 0.9])
+            shape_codes = np.concatenate([shape_codes, shape_codes])
+        vertices_parts = []
+        codes_parts = []
         for row in range(self.num_rows + 1):
             if row % 2 == 0:
                 cols = np.linspace(0, 1, self.num_rows + 1)
             else:
                 cols = np.linspace(offset / 2, 1 - offset / 2, self.num_rows)
             row_pos = row * offset
             for col_pos in cols:
-                vertices[cursor:cursor + shape_size] = (shape_vertices +
-                                                        (col_pos, row_pos))
-                codes[cursor:cursor + shape_size] = shape_codes
-                cursor += shape_size
-                if not self.filled:
-                    vertices[cursor:cursor + shape_size] = (inner_vertices +
-                                                            (col_pos, row_pos))
-                    codes[cursor:cursor + shape_size] = shape_codes
-                    cursor += shape_size
+                vertices_parts.append(shape_vertices + [col_pos, row_pos])
+                codes_parts.append(shape_codes)
+        np.concatenate(vertices_parts, out=vertices)
+        np.concatenate(codes_parts, out=codes)
 
 
 class Circles(Shapes):
@@ -149,16 +145,13 @@ def __init__(self, hatch, density):
         super().__init__(hatch, density)
 
 
-# TODO: __init__ and class attributes override all attributes set by
-# SmallCircles. Should this class derive from Circles instead?
-class SmallFilledCircles(SmallCircles):
+class SmallFilledCircles(Circles):
     size = 0.1
     filled = True
 
     def __init__(self, hatch, density):
         self.num_rows = (hatch.count('.')) * density
-        # Not super().__init__!
-        Circles.__init__(self, hatch, density)
+        super().__init__(hatch, density)
 
 
 class Stars(Shapes):