matplotlib · efiring · Apr 15, 2019 · Apr 9, 2019
diff --git a/doc/api/next_api_changes/2019-04-13-TH.rst b/doc/api/next_api_changes/2019-04-13-TH.rst
@@ -0,0 +1,7 @@
+API changes
+```````````
+
+`matplotlib.color.is_colorlike()` used to return True for all string
+representations of floats. However, only those with values in 0-1 are valid
+colors (representing grayscale values). ``is_colorlike()`` now returns False
+for string representations of floats outside 0-1.
diff --git a/lib/matplotlib/colors.py b/lib/matplotlib/colors.py
@@ -225,17 +225,23 @@ def _to_rgba_no_colorcycle(c, alpha=None):
            return tuple(color)
        # string gray.
        try:
-            return (float(c),) * 3 + (alpha if alpha is not None else 1.,)
+            c = float(c)
        except ValueError:
            pass
-        raise ValueError("Invalid RGBA argument: {!r}".format(orig_c))
+        else:
+            if not (0 <= c <= 1):
+                raise ValueError(
+                    f"Invalid string grayscale value {orig_c!r}. "
+                    f"Value must be within 0-1 range")
+            return c, c, c, alpha if alpha is not None else 1.
+        raise ValueError(f"Invalid RGBA argument: {orig_c!r}")
    # tuple color.
    c = np.array(c)
    if not np.can_cast(c.dtype, float, "same_kind") or c.ndim != 1:
        # Test the dtype explicitly as `map(float, ...)`, `np.array(...,
        # float)` and `np.array(...).astype(float)` all convert "0.5" to 0.5.
        # Test dimensionality to reject single floats.
-        raise ValueError("Invalid RGBA argument: {!r}".format(orig_c))
+        raise ValueError(f"Invalid RGBA argument: {orig_c!r}")
    # Return a tuple to prevent the cached value from being modified.
    c = tuple(c.astype(float))
    if len(c) not in [3, 4]:

diff --git a/lib/matplotlib/tests/test_colors.py b/lib/matplotlib/tests/test_colors.py
@@ -471,12 +471,6 @@ def test_autoscale_masked():
    plt.draw()


-def test_colors_no_float():
-    # Gray must be a string to distinguish 3-4 grays from RGB or RGBA.
-    with pytest.raises(ValueError):
-        mcolors.to_rgba(0.4)
-
-
 @image_comparison(baseline_images=['light_source_shading_topo'],
                  extensions=['png'])
 def test_light_source_topo_surface():
@@ -756,6 +750,18 @@ def test_conversions():
        hex_color


+def test_failed_conversions():
+    with pytest.raises(ValueError):
+        mcolors.to_rgba('5')
+    with pytest.raises(ValueError):
+        mcolors.to_rgba('-1')
+    with pytest.raises(ValueError):
+        mcolors.to_rgba('nan')
+    with pytest.raises(ValueError):
+        # Gray must be a string to distinguish 3-4 grays from RGB or RGBA.
+        mcolors.to_rgba(0.4)
+
+
 def test_grey_gray():
    color_mapping = mcolors._colors_full_map
    for k in color_mapping.keys():