Deprecate case-insensitive colors.

anntzer · anntzer · commit aec2a564331d · 2019-01-17T23:46:30.000+01:00
The motivation is explained in the changelog entry.

Note that even though

    plt.plot([1, 2], [3, 4], "Y")

"worked" so far,

    plt.plot([1, 2], [3, 4], "Yo")

(for example), i.e. additionally passing a marker spec, has not worked
at least since matplotlib 1.5 (a ValueError is thrown).  As this does
not appear to have ever been raised on the issue tracker, I believe the
number of users of UPPERCASE colors must be limited.
diff --git a/doc/api/next_api_changes/2019-01-19-AL.rst b/doc/api/next_api_changes/2019-01-19-AL.rst
@@ -0,0 +1,10 @@
+Deprecations
+````````````
+
+Support for passing colors as UPPERCASE strings is deprecated; color names will
+become case-sensitive (all-lowercase) after the deprecation period has passed.
+
+The goal is to decrease the number of ambiguous cases when using the ``data``
+keyword to plotting methods; e.g. ``plot("X", "Y", data={"X": ..., "Y": ...})``
+will not warn about "Y" possibly being a color anymore after the deprecation
+period has passed.
diff --git a/lib/matplotlib/colors.py b/lib/matplotlib/colors.py
@@ -194,9 +194,17 @@ def _to_rgba_no_colorcycle(c, alpha=None):
         # Named color.
         try:
             # This may turn c into a non-string, so we check again below.
-            c = _colors_full_map[c.lower()]
+            c = _colors_full_map[c]
         except KeyError:
-            pass
+            try:
+                c = _colors_full_map[c.lower()]
+            except KeyError:
+                pass
+            else:
+                cbook.warn_deprecated(
+                    "3.1", message="Support for case-insensitive colors is "
+                    "deprecated since Matplotlib %(since)s and will be "
+                    "removed %(removal)s.")
     if isinstance(c, str):
         # hex color with no alpha.
         match = re.match(r"\A#[a-fA-F0-9]{6}\Z", c)
diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
@@ -6036,3 +6036,11 @@ def test_annotate_across_transforms():
     ax.annotate("", xy=(x[150], y[150]), xycoords=ax.transData,
             xytext=(1, 0), textcoords=axins.transAxes,
             arrowprops=dict(arrowstyle="->"))
+
+
+def test_deprecated_uppercase_colors():
+    # Remove after end of deprecation period.
+    fig, ax = plt.subplots()
+    with pytest.warns(MatplotlibDeprecationWarning):
+        ax.plot([1, 2], color="B")
+        fig.canvas.draw()
