Update for checking whether colors have an alpha channel (matplotlib#27327)

* Update _has_alpha_channel

* Update lib/matplotlib/colors.py

Co-authored-by: Oscar Gustafsson <[email protected]>

* Clarify comment explanation

* Remove unnecessary check about is_color_like

* update docstring for undefined case

* Deprecate unused method

* Remove unnecessary method

* Flake formatting update

Co-authored-by: Tim Hoffmann <[email protected]>

* Don't specify what an undefined result does

Co-authored-by: Tim Hoffmann <[email protected]>

* Recursive check which handles hex colors too

Co-authored-by: Ruth Comer <[email protected]>

* Whitespace

* Remove is_color_like check and explain

Co-authored-by: Tim Hoffmann <[email protected]>

* Don't discuss undefined results

Co-authored-by: Tim Hoffmann <[email protected]>

* Remove test of undefined result

Co-authored-by: Tim Hoffmann <[email protected]>

* Spelling error

Co-authored-by: Tim Hoffmann <[email protected]>

* improved grammar in comment

---------

Co-authored-by: Oscar Gustafsson <[email protected]>
Co-authored-by: Tim Hoffmann <[email protected]>
Co-authored-by: Ruth Comer <[email protected]>

Author: 4 people

lib/matplotlib/colors.py

@@ -234,9 +234,35 @@ def is_color_like(c):
 
 
 def _has_alpha_channel(c):
-    """Return whether *c* is a color with an alpha channel."""
-    # 4-element sequences are interpreted as r, g, b, a
-    return not isinstance(c, str) and len(c) == 4
+    """
+    Return whether *c* is a color with an alpha channel.
+
+    If *c* is not a valid color specifier, then the result is undefined.
+    """
+    # The following logic uses the assumption that c is a valid color spec.
+    # For speed and simplicity, we intentionally don't care about other inputs.
+    # Anything can happen with them.
+
+    # if c is a hex, it has an alpha channel when it has 4 (or 8) digits after '#'
+    if isinstance(c, str):
+        if c[0] == '#' and (len(c) == 5 or len(c) == 9):
+            # example: '#fff8' or '#0f0f0f80'
+            return True
+    else:
+        # if c isn't a string, it can be an RGB(A) or a color-alpha tuple
+        # if it has length 4, it has an alpha channel
+        if len(c) == 4:
+            # example: [0.5, 0.5, 0.5, 0.5]
+            return True
+
+        # if it has length 2, it's a color/alpha tuple
+        # if the second element isn't None or the first element has length = 4
+        if len(c) == 2 and (c[1] is not None or _has_alpha_channel(c[0])):
+            # example: ([0.5, 0.5, 0.5, 0.5], None) or ('r', 0.5)
+            return True
+
+    # otherwise it doesn't have an alpha channel
+    return False
 
 
 def _check_color_like(**kwargs):

lib/matplotlib/tests/test_colors.py

@@ -1191,10 +1191,18 @@ def test_colormap_reversing(name):
 def test_has_alpha_channel():
     assert mcolors._has_alpha_channel((0, 0, 0, 0))
     assert mcolors._has_alpha_channel([1, 1, 1, 1])
+    assert mcolors._has_alpha_channel('#fff8')
+    assert mcolors._has_alpha_channel('#0f0f0f80')
+    assert mcolors._has_alpha_channel(('r', 0.5))
+    assert mcolors._has_alpha_channel(([1, 1, 1, 1], None))
     assert not mcolors._has_alpha_channel('blue')  # 4-char string!
     assert not mcolors._has_alpha_channel('0.25')
     assert not mcolors._has_alpha_channel('r')
     assert not mcolors._has_alpha_channel((1, 0, 0))
+    assert not mcolors._has_alpha_channel('#fff')
+    assert not mcolors._has_alpha_channel('#0f0f0f')
+    assert not mcolors._has_alpha_channel(('r', None))
+    assert not mcolors._has_alpha_channel(([1, 1, 1], None))
 
 
 def test_cn():