matplotlib · story645 · Mar 19, 2023 · Jan 30, 2023
diff --git a/doc/users/next_whats_new/new_color_spec_tuple.rst b/doc/users/next_whats_new/new_color_spec_tuple.rst
@@ -0,0 +1,21 @@
+Add a new valid color format ``(matplotlib_color, alpha)``
+----------------------------------------------------------
+
+
+.. plot::
+    :include-source: true
+
+    import matplotlib.pyplot as plt
+    from matplotlib.patches import Rectangle
+
+    fig, ax = plt.subplots()
+
+    rectangle = Rectangle((.2, .2), .6, .6,
+                          facecolor=('blue', 0.2),
+                          edgecolor=('green', 0.5))
+    ax.add_patch(rectangle)
+
+
+Users can define a color using the new color specification, *(matplotlib_color, alpha)*.
+Note that an explicit alpha keyword argument will override an alpha value from
+*(matplotlib_color, alpha)*.
diff --git a/galleries/examples/color/set_alpha.py b/galleries/examples/color/set_alpha.py
@@ -0,0 +1,53 @@
+"""
+=================================
+Ways to set a color's alpha value
+=================================
+
+Compare setting alpha by the *alpha* keyword argument and by one of the Matplotlib color
+formats. Often, the *alpha* keyword is the only tool needed to add transparency to a
+color. In some cases, the *(matplotlib_color, alpha)* color format provides an easy way
+to fine-tune the appearance of a Figure.
+
+"""
+
+import matplotlib.pyplot as plt
+import numpy as np
+
+# Fixing random state for reproducibility.
+np.random.seed(19680801)
+
+fig, (ax1, ax2) = plt.subplots(ncols=2, figsize=(8, 4))
+
+x_values = [n for n in range(20)]
+y_values = np.random.randn(20)
+
+facecolors = ['green' if y > 0 else 'red' for y in y_values]
+edgecolors = facecolors
+
+ax1.bar(x_values, y_values, color=facecolors, edgecolor=edgecolors, alpha=0.5)
+ax1.set_title("Explicit 'alpha' keyword value\nshared by all bars and edges")
+
+
+# Normalize y values to get distinct face alpha values.
+abs_y = [abs(y) for y in y_values]
+face_alphas = [n / max(abs_y) for n in abs_y]
+edge_alphas = [1 - alpha for alpha in face_alphas]
+
+colors_with_alphas = list(zip(facecolors, face_alphas))
+edgecolors_with_alphas = list(zip(edgecolors, edge_alphas))
+
+ax2.bar(x_values, y_values, color=colors_with_alphas,
+        edgecolor=edgecolors_with_alphas)
+ax2.set_title('Normalized alphas for\neach bar and each edge')
+
+plt.show()
+
+# %%
+#
+# .. admonition:: References
+#
+#    The use of the following functions, methods, classes and modules is shown
+#    in this example:
+#
+#    - `matplotlib.axes.Axes.bar`
+#    - `matplotlib.pyplot.subplots`
diff --git a/galleries/users_explain/colors/colors.py b/galleries/users_explain/colors/colors.py
@@ -68,6 +68,9 @@
 |           to black if cycle does not |                                      |
 |           include color.             |                                      |
 +--------------------------------------+--------------------------------------+
+| Tuple of one of the above color      | - ``('green', 0.3)``                 |
+| formats and an alpha float.          | - ``('#f00', 0.9)``                  |
++--------------------------------------+--------------------------------------+
 
 .. _xkcd color survey: https://xkcd.com/color/rgb/
 

diff --git a/lib/matplotlib/colors.py b/lib/matplotlib/colors.py
@@ -315,6 +315,13 @@ def _to_rgba_no_colorcycle(c, alpha=None):
     *alpha* is ignored for the color value ``"none"`` (case-insensitive),
     which always maps to ``(0, 0, 0, 0)``.
     """
+    if isinstance(c, tuple) and len(c) == 2:
+        if alpha is None:
+            c, alpha = c
+        else:
+            c = c[0]
+    if alpha is not None and not 0 <= alpha <= 1:
+        raise ValueError("'alpha' must be between 0 and 1, inclusive")
     orig_c = c
     if c is np.ma.masked:
         return (0., 0., 0., 0.)
@@ -425,6 +432,11 @@ def to_rgba_array(c, alpha=None):
         (n, 4) array of RGBA colors,  where each channel (red, green, blue,
         alpha) can assume values between 0 and 1.
     """
+    if isinstance(c, tuple) and len(c) == 2:
+        if alpha is None:
+            c, alpha = c
+        else:
+            c = c[0]
     # 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.)
@@ -464,9 +476,12 @@ def to_rgba_array(c, alpha=None):
             return np.array([to_rgba(c, a) for a in alpha], float)
         else:
             return np.array([to_rgba(c, alpha)], float)
-    except (ValueError, TypeError):
+    except TypeError:
         pass
-
+    except ValueError as e:
+        if e.args == ("'alpha' must be between 0 and 1, inclusive", ):
+            # ValueError is from _to_rgba_no_colorcycle().
+            raise e
     if isinstance(c, str):
         raise ValueError(f"{c!r} is not a valid color value.")
 

diff --git a/lib/matplotlib/tests/test_colors.py b/lib/matplotlib/tests/test_colors.py
@@ -1307,6 +1307,51 @@ def test_to_rgba_array_alpha_array():
     assert_array_equal(c[:, 3], alpha)
 
 
+def test_to_rgba_array_accepts_color_alpha_tuple():
+    assert_array_equal(
+        mcolors.to_rgba_array(('black', 0.9)),
+        [[0, 0, 0, 0.9]])
+
+
+def test_to_rgba_array_explicit_alpha_overrides_tuple_alpha():
+    assert_array_equal(
+        mcolors.to_rgba_array(('black', 0.9), alpha=0.5),
+        [[0, 0, 0, 0.5]])
+
+
+def test_to_rgba_array_accepts_color_alpha_tuple_with_multiple_colors():
+    color_array = np.array([[1., 1., 1., 1.], [0., 0., 1., 0.]])
+    assert_array_equal(
+        mcolors.to_rgba_array((color_array, 0.2)),
+        [[1., 1., 1., 0.2], [0., 0., 1., 0.2]])
+
+    color_sequence = [[1., 1., 1., 1.], [0., 0., 1., 0.]]
+    assert_array_equal(
+        mcolors.to_rgba_array((color_sequence, 0.4)),
+        [[1., 1., 1., 0.4], [0., 0., 1., 0.4]])
+
+
+def test_to_rgba_array_error_with_color_invalid_alpha_tuple():
+    with pytest.raises(ValueError, match="'alpha' must be between 0 and 1,"):
+        mcolors.to_rgba_array(('black', 2.0))
+
+
+@pytest.mark.parametrize('rgba_alpha',
+                         [('white', 0.5), ('#ffffff', 0.5), ('#ffffff00', 0.5),
+                          ((1.0, 1.0, 1.0, 1.0), 0.5)])
+def test_to_rgba_accepts_color_alpha_tuple(rgba_alpha):
+    assert mcolors.to_rgba(rgba_alpha) == (1, 1, 1, 0.5)
+
+
+def test_to_rgba_explicit_alpha_overrides_tuple_alpha():
+    assert mcolors.to_rgba(('red', 0.1), alpha=0.9) == (1, 0, 0, 0.9)
+
+
+def test_to_rgba_error_with_color_invalid_alpha_tuple():
+    with pytest.raises(ValueError, match="'alpha' must be between 0 and 1"):
+        mcolors.to_rgba(('blue', 2.0))
+
+
 def test_failed_conversions():
     with pytest.raises(ValueError):
         mcolors.to_rgba('5')