Support setting rcParams["image.cmap"] to Colormap instances.

anntzer · anntzer · commit bb2de5e02d89 · 2020-12-13T13:49:23.000+01:00
This makes it possible to set the rcParam to a non-registered colormap
(which cannot be referred to by name).  A use-case is to set it to e.g.
`viridis.with_extremes(bad="k")`.
diff --git a/doc/users/next_whats_new/2019-06-28-AL.rst b/doc/users/next_whats_new/2019-06-28-AL.rst
@@ -11,3 +11,7 @@ first copy the colormap and set the extreme colors on that copy.
 The new `.Colormap.set_extremes` method is provided for API symmetry with
 `.Colormap.with_extremes`, but note that it suffers from the same issue as the
 earlier individual setters.
+
+Additionally, it is now possible to set :rc:`image.cmap` to a `.Colormap`
+instance, such as a new colormap created with `~.Colormap.set_extremes`.  (This
+can only be done from Python code, not from the :file:`matplotlibrc` file.)
diff --git a/lib/matplotlib/rcsetup.py b/lib/matplotlib/rcsetup.py
@@ -25,7 +25,7 @@
 from matplotlib import _api, animation, cbook
 from matplotlib.cbook import ls_mapper
 from matplotlib.fontconfig_pattern import parse_fontconfig_pattern
-from matplotlib.colors import is_color_like
+from matplotlib.colors import Colormap, is_color_like
 
 # Don't let the original cycler collide with our validating cycler
 from cycler import Cycler, cycler as ccycler
@@ -393,6 +393,13 @@ def validate_color(s):
 
 validate_colorlist = _listify_validator(
     validate_color, allow_stringlist=True, doc='return a list of colorspecs')
+
+
+def _validate_cmap(s):
+    cbook._check_isinstance((str, Colormap), cmap=s)
+    return s
+
+
 validate_orientation = ValidateInStrings(
     'orientation', ['landscape', 'portrait'], _deprecated_since="3.3")
 
@@ -1141,7 +1148,7 @@ def _convert_validator_spec(key, conv):
 
     "image.aspect":          validate_aspect,  # equal, auto, a number
     "image.interpolation":   validate_string,
-    "image.cmap":            validate_string,  # gray, jet, etc.
+    "image.cmap":            _validate_cmap,  # gray, jet, etc.
     "image.lut":             validate_int,  # lookup table
     "image.origin":          ["upper", "lower"],
     "image.resample":        validate_bool,
