BUG: handle empty levels array in contour, closes #7486

efiring · efiring · commit 6ac197fd5840 · 2017-06-04T11:09:24.000-10:00
This relates to line contours only, not to filled contours.

This handles the case where an empty array is provided as the
"levels" kwarg, or where for any reason there are no levels within
the data range.  An example is the case of uniform z. The
levels array is then set to the minimum value of z so that
the plotting can proceed, and a warning is generated.
diff --git a/lib/matplotlib/contour.py b/lib/matplotlib/contour.py
@@ -1132,14 +1132,10 @@ def _autolev(self, N):
                 self.locator = ticker.LogLocator()
             else:
                 self.locator = ticker.MaxNLocator(N + 1, min_n_ticks=1)
-        zmax = self.zmax
-        zmin = self.zmin
-        lev = self.locator.tick_values(zmin, zmax)
+
+        lev = self.locator.tick_values(self.zmin, self.zmax)
         self._auto = True
-        if self.filled:
-            return lev
-        # For line contours, drop levels outside the data range.
-        return lev[(lev > zmin) & (lev < zmax)]
+        return lev
 
     def _contour_level_args(self, z, args):
         """
@@ -1165,6 +1161,17 @@ def _contour_level_args(self, z, args):
                         "Last {0} arg must give levels; see help({0})"
                         .format(fn))
             self.levels = lev
+        else:
+            self.levels = np.asarray(self.levels).astype(np.float64)
+
+        if not self.filled:
+            inside = (self.levels > self.zmin) & (self.levels < self.zmax)
+            self.levels = self.levels[inside]
+            if len(self.levels) == 0:
+                self.levels = [self.zmin]
+                warnings.warn("No contour levels were found"
+                              " within the data range.")
+
         if self.filled and len(self.levels) < 2:
             raise ValueError("Filled contours require at least 2 levels.")
 
diff --git a/lib/matplotlib/tests/test_contour.py b/lib/matplotlib/tests/test_contour.py
@@ -132,6 +132,28 @@ def test_contour_shape_invalid_2():
     excinfo.match(r'Input z must be a 2D array.')
 
 
+def test_contour_empty_levels():
+
+    x = np.arange(9)
+    z = np.random.random((9, 9))
+
+    fig, ax = plt.subplots()
+    with pytest.warns(UserWarning) as record:
+        ax.contour(x, x, z, levels=[])
+    assert len(record) == 1
+
+
+def test_contour_uniform_z():
+
+    x = np.arange(9)
+    z = np.ones((9, 9))
+
+    fig, ax = plt.subplots()
+    with pytest.warns(UserWarning) as record:
+        ax.contour(x, x, z)
+    assert len(record) == 1
+
+
 @image_comparison(baseline_images=['contour_manual_labels'])
 def test_contour_manual_labels():
 
