Remove contour warning for "no-valid-levels".

If the user explicitly passes a levels array (the default is
auto-determined), let's assume that they know what they are doing.

Author: anntzer

doc/api/next_api_changes/behavior/24912-AL.rst

@@ -0,0 +1,4 @@
+``contour`` no longer warns if no contour lines are drawn.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+This can occur if the user explicitly passes a ``levels`` array with no values
+between ``z.min()`` and ``z.max()``; or if ``z`` has the same value everywhere.

lib/matplotlib/contour.py

@@ -1137,18 +1137,8 @@ def _process_contour_level_args(self, args, z_dtype):
             self.levels = self._autolev(levels_arg)
         else:
             self.levels = np.asarray(levels_arg, np.float64)
-
-        if not self.filled:
-            inside = (self.levels > self.zmin) & (self.levels < self.zmax)
-            levels_in = self.levels[inside]
-            if len(levels_in) == 0:
-                self.levels = [self.zmin]
-                _api.warn_external(
-                    "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.")
-
         if len(self.levels) > 1 and np.min(np.diff(self.levels)) <= 0.0:
             raise ValueError("Contour levels must be increasing")
 

lib/matplotlib/tests/test_contour.py

@@ -62,15 +62,16 @@ def test_contour_shape_error(args, message):
         ax.contour(*args)
 
 
-def test_contour_empty_levels():
-
-    x = np.arange(9)
-    z = np.random.random((9, 9))
-
+def test_contour_no_valid_levels():
     fig, ax = plt.subplots()
-    with pytest.warns(UserWarning) as record:
-        ax.contour(x, x, z, levels=[])
-    assert len(record) == 1
+    # no warning for empty levels.
+    ax.contour(np.random.rand(9, 9), levels=[])
+    # no warning if levels is given and is not within the range of z.
+    cs = ax.contour(np.arange(81).reshape((9, 9)), levels=[100])
+    # ... and if fmt is given.
+    ax.clabel(cs, fmt={100: '%1.2f'})
+    # no warning if z is uniform.
+    ax.contour(np.ones((9, 9)))
 
 
 def test_contour_Nlevels():
@@ -84,33 +85,6 @@ def test_contour_Nlevels():
     assert (cs1.levels == cs2.levels).all()
 
 
-def test_contour_badlevel_fmt():
-    # Test edge case from https://github.com/matplotlib/matplotlib/issues/9742
-    # User supplied fmt for each level as a dictionary, but Matplotlib changed
-    # the level to the minimum data value because no contours possible.
-    # This was fixed in https://github.com/matplotlib/matplotlib/pull/9743
-    x = np.arange(9)
-    z = np.zeros((9, 9))
-
-    fig, ax = plt.subplots()
-    fmt = {1.: '%1.2f'}
-    with pytest.warns(UserWarning) as record:
-        cs = ax.contour(x, x, z, levels=[1.])
-        ax.clabel(cs, fmt=fmt)
-    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(['contour_manual_labels'], remove_text=True, style='mpl20')
 def test_contour_manual_labels():
     x, y = np.meshgrid(np.arange(0, 10), np.arange(0, 10))