Downgrade exception on mapping list length mismatch to warning (#2856)

* Downgrade exception on mapping list length mismatch to warning

* Lint

* Fix pairplot test

* Set stacklevel to report warning in user code

seaborn/_oldcore.py

@@ -55,6 +55,30 @@ def map(cls, plotter, *args, **kwargs):
         setattr(plotter, method_name, cls(plotter, *args, **kwargs))
         return plotter
 
+    def _check_list_length(self, levels, values, variable):
+        """Input check when values are provided as a list."""
+        # Copied from _core/properties; eventually will be replaced for that.
+        message = ""
+        if len(levels) > len(values):
+            message = " ".join([
+                f"\nThe {variable} list has fewer values ({len(values)})",
+                f"than needed ({len(levels)}) and will cycle, which may",
+                "produce an uninterpretable plot."
+            ])
+            values = [x for _, x in zip(levels, itertools.cycle(values))]
+
+        elif len(values) > len(levels):
+            message = " ".join([
+                f"The {variable} list has more values ({len(values)})",
+                f"than needed ({len(levels)}), which may not be intended.",
+            ])
+            values = values[:len(levels)]
+
+        if message:
+            warnings.warn(message, UserWarning, stacklevel=6)
+
+        return values
+
     def _lookup_single(self, key):
         """Apply the mapping to a single data value."""
         return self.lookup_table[key]
@@ -212,10 +236,7 @@ def categorical_mapping(self, data, palette, order):
                 else:
                     colors = color_palette("husl", n_colors)
             elif isinstance(palette, list):
-                if len(palette) != n_colors:
-                    err = "The palette list has the wrong number of colors."
-                    raise ValueError(err)
-                colors = palette
+                colors = self._check_list_length(levels, palette, "palette")
             else:
                 colors = color_palette(palette, n_colors)
 
@@ -367,10 +388,7 @@ def categorical_mapping(self, data, sizes, order):
         elif isinstance(sizes, list):
 
             # List inputs give size values in the same order as the levels
-            if len(sizes) != len(levels):
-                err = "The `sizes` list has the wrong number of values."
-                raise ValueError(err)
-
+            sizes = self._check_list_length(levels, sizes, "sizes")
             lookup_table = dict(zip(levels, sizes))
 
         else:
@@ -578,9 +596,7 @@ def _map_attributes(self, arg, levels, defaults, attr):
                 raise ValueError(err)
             lookup_table = arg
         elif isinstance(arg, Sequence):
-            if len(levels) != len(arg):
-                err = f"The `{attr}` argument has the wrong number of values"
-                raise ValueError(err)
+            arg = self._check_list_length(levels, arg, attr)
             lookup_table = dict(zip(levels, arg))
         elif arg:
             err = f"This `{attr}` argument was not understood: {arg}"

tests/test_axisgrid.py

@@ -1385,7 +1385,7 @@ def test_pairplot_markers(self):
         m2 = g._legend.legendHandles[1].get_paths()[0]
         assert m1 != m2
 
-        with pytest.raises(ValueError):
+        with pytest.warns(UserWarning):
             g = ag.pairplot(self.df, hue="a", vars=vars, markers=markers[:-2])
 
     def test_corner_despine(self):

tests/test_core.py

@@ -151,7 +151,7 @@ def test_hue_map_categorical(self, wide_df, long_df):
 
         # Test list with wrong number of colors
         palette = colors[:-1]
-        with pytest.raises(ValueError):
+        with pytest.warns(UserWarning):
             HueMapping(p, palette=palette)
 
         # Test hue order
@@ -296,7 +296,7 @@ def test_hue_map_numeric(self, long_df):
         assert m.lookup_table == dict(zip(hue_levels, palette))
 
         palette = color_palette("Blues", len(hue_levels) + 1)
-        with pytest.raises(ValueError):
+        with pytest.warns(UserWarning):
             HueMapping(p, palette=palette)
 
         # Test dictionary of colors
@@ -460,7 +460,7 @@ def test_map_size_categorical(self, long_df):
 
         # Test sizes list with wrong length
         sizes = list(np.random.rand(len(levels) + 1))
-        with pytest.raises(ValueError):
+        with pytest.warns(UserWarning):
             SizeMapping(p, sizes=sizes)
 
         # Test sizes dict with missing levels
@@ -578,13 +578,13 @@ def test_map_style(self, long_df):
             assert_array_equal(m(key, "path").vertices, path.vertices)
 
         # Test too many levels with style lists
-        with pytest.raises(ValueError):
+        with pytest.warns(UserWarning):
             StyleMapping(p, markers=["o", "s"], dashes=False)
 
-        with pytest.raises(ValueError):
+        with pytest.warns(UserWarning):
             StyleMapping(p, markers=False, dashes=[(2, 1)])
 
-        # Test too many levels with style dicts
+        # Test missing keys with style dicts
         markers, dashes = {"a": "o", "b": "s"}, False
         with pytest.raises(ValueError):
             StyleMapping(p, markers=markers, dashes=dashes)