fix: minor StepPatch adjustments

andrzejnovak · andrzejnovak · commit ecc54cb11180 · 2020-09-16T11:06:05.000+02:00
diff --git a/doc/users/next_whats_new/steppatch_and_stairs.rst b/doc/users/next_whats_new/steppatch_and_stairs.rst
@@ -1,5 +1,6 @@
-New `~.matplotlib.patches.StepPatch` artist and a `.pyplot.stairs` method
--------------------------------------------------------------------------
+New StepPatch artist and a stairs method
+----------------------------------------
+New `~.matplotlib.patches.StepPatch` artist and  `.pyplot.stairs` method.
 For both the artist and the function, the x-like edges input is one 
 longer than the y-like values input
 
diff --git a/lib/matplotlib/patches.py b/lib/matplotlib/patches.py
@@ -1028,9 +1028,8 @@ def _update_path(self):
         if self._edges.size - 1 != self._values.size:
             raise ValueError('Size mismatch between "values" and "edges". '
                              "Expected `len(values) + 1 == len(edges)`, but "
-                             "they are or lengths {} and {}".format(
-                                 self._edges.size, self._values.size)
-                             )
+                             f"`len(values) = {self._values.size}` and "
+                             f"`len(edges) = {self._edges.size}`.")
         verts, codes = [], []
         for idx0, idx1 in cbook.contiguous_regions(~np.isnan(self._values)):
             x = np.repeat(self._edges[idx0:idx1+1], 2)
diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
@@ -1847,14 +1847,14 @@ def test_stairs_invalid_mismatch():
 
 @pytest.mark.xfail
 def test_stairs_invalid_update():
-    h = plt.stairs([1, 2], [0, 1])
-    h.set_edges(np.arange(5))
+    h = plt.stairs([1, 2], [0, 1, 2])
+    h.set_edges([1, np.nan, 2])
 
 
 @pytest.mark.xfail
 def test_stairs_invalid_update2():
-    h = plt.stairs([1, 2], [0, 1])
-    h.set_edges([1, np.nan, 2])
+    h = plt.stairs([1, 2], [0, 1, 2])
+    h.set_edges(np.arange(5))
 
 
 @image_comparison(['test_stairs_options.png'], remove_text=True)
