Fixed bug in find_nearest_contour; Added test

lib/matplotlib/contour.py

@@ -1334,6 +1334,8 @@ def find_nearest_contour(self, x, y, indices=None, pixel=True):
         """
         Find the point in the contour plot that is closest to ``(x, y)``.
 
+        This method does not support filled contours.
+
         Parameters
         ----------
         x, y : float
@@ -1370,8 +1372,11 @@ def find_nearest_contour(self, x, y, indices=None, pixel=True):
         # sufficiently well that the time is not noticeable.
         # Nonetheless, improvements could probably be made.
 
+        if self.filled:
+            raise ValueError("Method does not support filled contours.")
+
         if indices is None:
-            indices = range(len(self.levels))
+            indices = range(len(self.collections))
 
         d2min = np.inf
         conmin = None

lib/matplotlib/tests/test_contour.py

@@ -470,6 +470,48 @@ def test_contour_line_start_on_corner_edge():
     cbar.add_lines(lines)
 
 
+def test_find_nearest_contour():
+    xy = np.indices((15, 15))
+    img = np.exp(-np.pi * (np.sum((xy - 5)**2, 0)/5.**2))
+    cs = plt.contour(img, 10)
+
+    nearest_contour = cs.find_nearest_contour(1, 1, pixel=False)
+    expected_nearest = (1, 0, 33, 1.965966, 1.965966, 1.866183)
+    assert_array_almost_equal(nearest_contour, expected_nearest)
+
+    nearest_contour = cs.find_nearest_contour(8, 1, pixel=False)
+    expected_nearest = (1, 0, 5, 7.550173, 1.587542, 0.547550)
+    assert_array_almost_equal(nearest_contour, expected_nearest)
+
+    nearest_contour = cs.find_nearest_contour(2, 5, pixel=False)
+    expected_nearest = (3, 0, 21, 1.884384, 5.023335, 0.013911)
+    assert_array_almost_equal(nearest_contour, expected_nearest)
+
+    nearest_contour = cs.find_nearest_contour(2, 5,
+                                              indices=(5, 7),
+                                              pixel=False)
+    expected_nearest = (5, 0, 16, 2.628202, 5.0, 0.394638)
+    assert_array_almost_equal(nearest_contour, expected_nearest)
+
+
+def test_find_nearest_contour_no_filled():
+    xy = np.indices((15, 15))
+    img = np.exp(-np.pi * (np.sum((xy - 5)**2, 0)/5.**2))
+    cs = plt.contourf(img, 10)
+
+    with pytest.raises(ValueError,
+                       match="Method does not support filled contours."):
+        cs.find_nearest_contour(1, 1, pixel=False)
+
+    with pytest.raises(ValueError,
+                       match="Method does not support filled contours."):
+        cs.find_nearest_contour(1, 10, indices=(5, 7), pixel=False)
+
+    with pytest.raises(ValueError,
+                       match="Method does not support filled contours."):
+        cs.find_nearest_contour(2, 5, indices=(2, 7), pixel=True)
+
+
 @mpl.style.context("default")
 def test_contour_autolabel_beyond_powerlimits():
     ax = plt.figure().add_subplot()