Fix QuadContourSet breaking change becoming Artist subclass

Solves GitHub issue #594.

Author: molinav

CHANGELOG.md

@@ -72,6 +72,10 @@ https://semver.org/spec/v2.0.0.html
   Clang 16 and GCC 14 (PR [#595] by @fweimer-rh).
 - Apply basic cleanup of `_geoslib.pyx` source code (i.e. basic linting,
   removal of commented code, version update).
+- Fix breaking change from `matplotlib` 3.8 due to the promotion of
+  `QuadContourSet` objects into `Artist` objects, which affected
+  `Basemap.contour`, `Basemap.contourf` and `Basemap.nightshade`
+  (solves issue [#594]).
 
 ### Removed
 - Attribute `__version__` in `mpl_toolkits.basemap.proj` module.
@@ -1077,6 +1081,8 @@ https://semver.org/spec/v2.0.0.html
 
 [#595]:
 https://github.com/matplotlib/basemap/pull/595
+[#594]
+https://github.com/matplotlib/basemap/issues/594
 [#593]:
 https://github.com/matplotlib/basemap/pull/593
 [#592]:

packages/basemap/src/mpl_toolkits/basemap/__init__.py

@@ -31,6 +31,7 @@
 import numpy.ma as ma
 
 import matplotlib as mpl
+from matplotlib.artist import Artist
 from matplotlib.collections import LineCollection
 from matplotlib.collections import PolyCollection
 from matplotlib.image import imread
@@ -3592,7 +3593,11 @@ def contour(self,x,y,data,*args,**kwargs):
         # set axes limits to fit map region.
         self.set_axes_limits(ax=ax)
         # clip to map limbs
-        CS.collections,c = self._cliplimb(ax,CS.collections)
+        if isinstance(CS, Artist):
+            # Since MPL 3.8, `QuadContourSet` objects are `Artist` objects too.
+            CS, c = self._cliplimb(ax, CS)
+        else:
+            CS.collections, c = self._cliplimb(ax, CS.collections)
         return CS
 
     @_transform
@@ -3688,7 +3693,11 @@ def contourf(self,x,y,data,*args,**kwargs):
         # set axes limits to fit map region.
         self.set_axes_limits(ax=ax)
         # clip to map limbs
-        CS.collections,c = self._cliplimb(ax,CS.collections)
+        if isinstance(CS, Artist):
+            # Since MPL 3.8, `QuadContourSet` objects are `Artist` objects too.
+            CS, c = self._cliplimb(ax, CS)
+        else:
+            CS.collections, c = self._cliplimb(ax, CS.collections)
         return CS
 
     @_transformuv
@@ -4733,12 +4742,18 @@ def nightshade(self,date,color="k",delta=0.25,alpha=0.5,ax=None,zorder=2):
         # contour the day-night grid, coloring the night area
         # with the specified color and transparency.
         CS = self.contourf(x,y,daynight,1,colors=[color],alpha=alpha,ax=ax)
-        # set zorder on ContourSet collections show night shading
-        # is on top.
-        for c in CS.collections:
-            c.set_zorder(zorder)
-        # clip to map limbs
-        CS.collections,c = self._cliplimb(ax,CS.collections)
+        if isinstance(CS, Artist):
+            # Since MPL 3.8, `QuadContourSet` objects are `Artist` objects too.
+            CS.set_zorder(zorder)
+            # clip to map limbs
+            CS, c = self._cliplimb(ax, CS)
+        else:
+            # set zorder on ContourSet collections show night shading
+            # is on top.
+            for c in CS.collections:
+                c.set_zorder(zorder)
+            # clip to map limbs
+            CS.collections, c = self._cliplimb(ax, CS.collections)
         return CS
 
     def _check_ax(self):