Support make_compound_path concatenating only empty paths.

anntzer · anntzer · commit 48ea00f47177 · 2023-02-18T21:24:32.000+01:00
Previously `codes[i]` would fail if codes (i.e. the concatenated path)
had length zero.
diff --git a/lib/matplotlib/path.py b/lib/matplotlib/path.py
@@ -318,29 +318,25 @@ def make_compound_path_from_polys(cls, XY):
 
     @classmethod
     def make_compound_path(cls, *args):
+        r"""
+        Concatenate a list of `Path`\s into a single `.Path`, removing all `.STOP`\s.
         """
-        Make a compound path from a list of `Path` objects. Blindly removes
-        all `Path.STOP` control points.
-        """
-        # Handle an empty list in args (i.e. no args).
         if not args:
             return Path(np.empty([0, 2], dtype=np.float32))
-        vertices = np.concatenate([x.vertices for x in args])
+        vertices = np.concatenate([path.vertices for path in args])
         codes = np.empty(len(vertices), dtype=cls.code_type)
         i = 0
         for path in args:
+            size = len(path.vertices)
             if path.codes is None:
-                codes[i] = cls.MOVETO
-                codes[i + 1:i + len(path.vertices)] = cls.LINETO
+                if size:
+                    codes[i] = cls.MOVETO
+                    codes[i+1:i+size] = cls.LINETO
             else:
-                codes[i:i + len(path.codes)] = path.codes
-            i += len(path.vertices)
-        # remove STOP's, since internal STOPs are a bug
-        not_stop_mask = codes != cls.STOP
-        vertices = vertices[not_stop_mask, :]
-        codes = codes[not_stop_mask]
-
-        return cls(vertices, codes)
+                codes[i:i+size] = path.codes
+            i += size
+        not_stop_mask = codes != cls.STOP  # Remove STOPs, as internal STOPs are a bug.
+        return cls(vertices[not_stop_mask], codes[not_stop_mask])
 
     def __repr__(self):
         return f"Path({self.vertices!r}, {self.codes!r})"
diff --git a/lib/matplotlib/tests/test_path.py b/lib/matplotlib/tests/test_path.py
@@ -205,6 +205,9 @@ def test_make_compound_path_empty():
     # This makes it easier to write generic path based code.
     r = Path.make_compound_path()
     assert r.vertices.shape == (0, 2)
+    r2 = Path.make_compound_path(r, r)
+    assert r2.vertices.shape == (0, 2)
+    assert r2.codes.shape == (0,)
 
 
 def test_make_compound_path_stops():
