Legacy contouring only warns if levels decreasing

Author: ianthomas23

lib/matplotlib/contour.py

@@ -1191,8 +1191,12 @@ def _contour_level_args(self, z, args):
             self.levels = lev
         if self.filled and len(self.levels) < 2:
             raise ValueError("Filled contours require at least 2 levels.")
+
         if len(self.levels) > 1 and np.amin(np.diff(self.levels)) <= 0.0:
-            raise ValueError("Contour levels must be increasing")
+            if hasattr(self, '_corner_mask') and self._corner_mask == 'legacy':
+                warnings.warn("Contour levels are not increasing")
+            else:
+                raise ValueError("Contour levels must be increasing")
 
     def _process_levels(self):
         """
@@ -1444,16 +1448,16 @@ def _process_args(self, *args, **kwargs):
             else:
                 contour_generator = args[0]._contour_generator
         else:
+            self._corner_mask = kwargs.get('corner_mask', None)
+            if self._corner_mask is None:
+                self._corner_mask = mpl.rcParams['contour.corner_mask']
+
             x, y, z = self._contour_args(args, kwargs)
 
             _mask = ma.getmask(z)
             if _mask is ma.nomask or not _mask.any():
                 _mask = None
 
-            self._corner_mask = kwargs.get('corner_mask', None)
-            if self._corner_mask is None:
-                self._corner_mask = mpl.rcParams['contour.corner_mask']
-
             if self._corner_mask == 'legacy':
                 cbook.warn_deprecated('1.5',
                                       name="corner_mask='legacy'",

lib/matplotlib/tests/test_contour.py

@@ -9,7 +9,8 @@
 from matplotlib import mlab
 from matplotlib.testing.decorators import cleanup, image_comparison
 from matplotlib import pyplot as plt
-from nose.tools import assert_raises
+from nose.tools import assert_equal, assert_raises
+import warnings
 
 import re
 
@@ -271,6 +272,11 @@ def test_contourf_decreasing_levels():
     z = [[0.1, 0.3], [0.5, 0.7]]
     plt.figure()
     assert_raises(ValueError, plt.contourf, z, [1.0, 0.0])
+    # Legacy contouring algorithm gives a warning rather than raising an error,
+    # plus a DeprecationWarning.
+    with warnings.catch_warnings(record=True) as w:
+        plt.contourf(z, [1.0, 0.0], corner_mask='legacy')
+    assert_equal(len(w), 2)
 
 
 if __name__ == '__main__':