matplotlib · QuLogic · Jul 22, 2020 · Jul 22, 2020
diff --git a/lib/matplotlib/patches.py b/lib/matplotlib/patches.py
@@ -1063,13 +1063,27 @@ def set_xy(self, xy):
         ----------
         xy : (N, 2) array-like
             The coordinates of the vertices.
+
+        Notes
+        -----
+        Unlike `~.path.Path`, we do not ignore the last input vertex. If the
+        polygon is meant to be closed, and the last point of the polygon is not
+        equal to the first, we assume that the user has not explicitly passed a
+        ``CLOSEPOLY`` vertex, and add it ourselves.
         """
         xy = np.asarray(xy)
+        nverts, _ = xy.shape
         if self._closed:
-            if len(xy) and (xy[0] != xy[-1]).any():
+            # if the first and last vertex are the "same", then we assume that
+            # the user explicitly passed the CLOSEPOLY vertex. Otherwise, we
+            # have to append one since the last vertex will be "ignored" by
+            # Path
+            if nverts == 1 or nverts > 1 and (xy[0] != xy[-1]).any():
                 xy = np.concatenate([xy, [xy[0]]])
         else:
-            if len(xy) > 2 and (xy[0] == xy[-1]).all():
+            # if we aren't closed, and the last vertex matches the first, then
+            # we assume we have an unecessary CLOSEPOLY vertex and remove it
+            if nverts > 2 and (xy[0] == xy[-1]).all():
                 xy = xy[:-1]
         self._path = Path(xy, closed=self._closed)
         self.stale = True

diff --git a/lib/matplotlib/tests/test_patches.py b/lib/matplotlib/tests/test_patches.py
@@ -7,6 +7,7 @@
 
 from matplotlib.patches import Polygon, Rectangle, FancyArrowPatch
 from matplotlib.testing.decorators import image_comparison, check_figures_equal
+from matplotlib.transforms import Bbox
 import matplotlib.pyplot as plt
 from matplotlib import (
     collections as mcollections, colors as mcolors, patches as mpatches,
@@ -556,3 +557,9 @@ def test_rotated_arcs():
         ax.axvline(0, color="k")
         ax.set_axis_off()
         ax.set_aspect("equal")
+
+
+def test_degenerate_polygon():
+    point = [0, 0]
+    correct_extents = Bbox([point, point]).extents
+    assert np.all(Polygon([point]).get_extents().extents == correct_extents)