From a3f0abcebf6514f3e37a6812f76b0d7562ba0ecd Mon Sep 17 00:00:00 2001
From: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>
Date: Sat, 15 Jan 2022 01:06:06 +0100
Subject: [PATCH] Partial fix for grid alpha

---
 lib/matplotlib/axis.py              | 10 ++++++++--
 lib/matplotlib/colors.py            |  6 ++++++
 lib/matplotlib/tests/test_colors.py |  9 +++++++++
 3 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/lib/matplotlib/axis.py b/lib/matplotlib/axis.py
index 07f15a87b1bc..ba3d543441eb 100644
--- a/lib/matplotlib/axis.py
+++ b/lib/matplotlib/axis.py
@@ -12,6 +12,7 @@
 from matplotlib import _api
 import matplotlib.artist as martist
 import matplotlib.cbook as cbook
+import matplotlib.colors as mcolors
 import matplotlib.lines as mlines
 import matplotlib.scale as mscale
 import matplotlib.text as mtext
@@ -143,7 +144,12 @@ def __init__(
             grid_linestyle = mpl.rcParams["grid.linestyle"]
         if grid_linewidth is None:
             grid_linewidth = mpl.rcParams["grid.linewidth"]
-        if grid_alpha is None:
+        if grid_alpha is None and not mcolors._has_alpha_channel(grid_color):
+            # alpha precedence: kwarg > color alpha > rcParams['grid.alpha']
+            # Note: only resolve to rcParams if the color does not have alpha
+            # otherwise `grid(color=(1, 1, 1, 0.5))` would work like
+            #   grid(color=(1, 1, 1, 0.5), alpha=rcParams['grid.alpha'])
+            # so the that the rcParams default would override color alpha.
             grid_alpha = mpl.rcParams["grid.alpha"]
         grid_kw = {k[5:]: v for k, v in kwargs.items()}
 
@@ -1413,7 +1419,7 @@ def grid(self, visible=None, which='major', **kwargs):
                 visible = True
         which = which.lower()
         _api.check_in_list(['major', 'minor', 'both'], which=which)
-        gridkw = {'grid_' + item[0]: item[1] for item in kwargs.items()}
+        gridkw = {f'grid_{name}': value for name, value in kwargs.items()}
         if which in ['minor', 'both']:
             gridkw['gridOn'] = (not self._minor_tick_kw['gridOn']
                                 if visible is None else visible)
diff --git a/lib/matplotlib/colors.py b/lib/matplotlib/colors.py
index d3e53e0ef01b..189e75758f30 100644
--- a/lib/matplotlib/colors.py
+++ b/lib/matplotlib/colors.py
@@ -121,6 +121,12 @@ def is_color_like(c):
         return True
 
 
+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
+
+
 def _check_color_like(**kwargs):
     """
     For each *key, value* pair in *kwargs*, check that *value* is color-like.
diff --git a/lib/matplotlib/tests/test_colors.py b/lib/matplotlib/tests/test_colors.py
index bedff6341af1..0db62f82b64e 100644
--- a/lib/matplotlib/tests/test_colors.py
+++ b/lib/matplotlib/tests/test_colors.py
@@ -1166,6 +1166,15 @@ def test_colormap_reversing(name):
     assert_array_almost_equal(cmap(np.nan), cmap_r(np.nan))
 
 
+def test_has_alpha_channel():
+    assert mcolors._has_alpha_channel((0, 0, 0, 0))
+    assert mcolors._has_alpha_channel([1, 1, 1, 1])
+    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))
+
+
 def test_cn():
     matplotlib.rcParams['axes.prop_cycle'] = cycler('color',
                                                     ['blue', 'r'])