ENH: Accept ('color', alpha) tuple to define a color

j1642 · j1642 · commit 9b308458cf59 · 2023-01-30T13:04:39.000-05:00
diff --git a/lib/matplotlib/colors.py b/lib/matplotlib/colors.py
@@ -286,6 +286,11 @@ def to_rgba(c, alpha=None):
         Tuple of floats ``(r, g, b, a)``, where each channel (red, green, blue,
         alpha) can assume values between 0 and 1.
     """
+    if isinstance(c, tuple) and len(c) == 2 and isinstance(c[0], str):
+        if 0 <= c[1] <= 1:
+            c, alpha = c[0], c[1]
+        else:
+            raise ValueError('Alpha must be between 0 and 1, inclusive.')
     # Special-case nth color syntax because it should not be cached.
     if _is_nth_color(c):
         prop_cycler = mpl.rcParams['axes.prop_cycle']
@@ -428,6 +433,11 @@ def to_rgba_array(c, alpha=None):
     # Special-case inputs that are already arrays, for performance.  (If the
     # array has the wrong kind or shape, raise the error during one-at-a-time
     # conversion.)
+    if isinstance(c, tuple) and len(c) == 2 and isinstance(c[0], str):
+        if 0 <= c[1] <= 1:
+            c, alpha = c[0], c[1]
+        else:
+            raise ValueError('Alpha must be between 0 and 1, inclusive.')
     if np.iterable(alpha):
         alpha = np.asarray(alpha).ravel()
     if (isinstance(c, np.ndarray) and c.dtype.kind in "if"
diff --git a/lib/matplotlib/tests/test_colors.py b/lib/matplotlib/tests/test_colors.py
@@ -1301,6 +1301,21 @@ def test_to_rgba_array_alpha_array():
     assert_array_equal(c[:, 3], alpha)
 
 
+def test_to_rgba_array_color_alpha_tuple():
+    booleans = mcolors.to_rgba_array(('red', 0.1)) \
+               == mcolors.to_rgba_array('red', alpha=0.1)
+    assert booleans.all()
+
+
+def test_to_rgba_array_raises_error():
+    with pytest.raises(ValueError, match='Alpha must be between 0 and 1,'):
+        mcolors.to_rgba_array(('blue', 2))
+
+
+def test_to_rgba_color_alpha_tuple():
+    assert mcolors.to_rgba(('red', 0.1)) == mcolors.to_rgba('red', alpha=0.1)
+
+
 def test_failed_conversions():
     with pytest.raises(ValueError):
         mcolors.to_rgba('5')
@@ -1313,6 +1328,8 @@ def test_failed_conversions():
     with pytest.raises(ValueError):
         # Gray must be a string to distinguish 3-4 grays from RGB or RGBA.
         mcolors.to_rgba(0.4)
+    with pytest.raises(ValueError, match='Alpha must be between 0 and 1,'):
+        mcolors.to_rgba(('blue', 2))
 
 
 def test_grey_gray():
