[MNT]: Turn ContourSet into a (nearly) plain Collection #25128

diff --git a/doc/api/next_api_changes/deprecations/XXXXX-AL.rst b/doc/api/next_api_changes/deprecations/XXXXX-AL.rst
new file mode 100644
index 0000000000..caf6506209
--- /dev/null
+++ b/doc/api/next_api_changes/deprecations/XXXXX-AL.rst
@@ -0,0 +1,4 @@
+``QuadContourSet.allsegs`` and ``QuadContourSet.allkinds``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+These attributes are deprecated; directly retrieve the vertices and codes of
+the Path objects in ``QuadContourSet.collections`` if necessary.
diff --git a/lib/matplotlib/axes/_base.py b/lib/matplotlib/axes/_base.py
index db25af57ac..d46238bd97 100644
--- a/lib/matplotlib/axes/_base.py
+++ b/lib/matplotlib/axes/_base.py
@@ -2179,10 +2179,7 @@ class _AxesBase(martist.Artist):
         _api.check_isinstance(
             (mpl.contour.ContourSet, mcoll.Collection, mimage.AxesImage),
             im=im)
-        if isinstance(im, mpl.contour.ContourSet):
-            if im.collections[0] not in self._children:
-                raise ValueError("ContourSet must be in current Axes")
-        elif im not in self._children:
+        if im not in self._children:
             raise ValueError("Argument must be an image, collection, or "
                              "ContourSet in this Axes")
         self._current_image = im
diff --git a/lib/matplotlib/contour.py b/lib/matplotlib/contour.py
index 060d9990b0..f24b888cc0 100644
--- a/lib/matplotlib/contour.py
+++ b/lib/matplotlib/contour.py
@@ -3,7 +3,6 @@ Classes to support contour plotting and labelling for the Axes class.
 """
 
 import functools
-import itertools
 from numbers import Integral
 
 import numpy as np
@@ -474,6 +473,7 @@ class ContourLabeler:
 
         # calc_label_rot_and_inline() requires that (xmin, ymin)
         # be a vertex in the path. So, if it isn't, add a vertex here
+        # FIXME: Adapt to now API.
         paths = self.collections[conmin].get_paths()  # paths of correct coll.
         lc = paths[segmin].vertices  # vertices of correct segment
         # Where should the new vertex be added in data-units?
@@ -524,8 +524,9 @@ class ContourLabeler:
                 self.labelCValueList,
         )):
 
+            # FIXME: Adapt to new API.
             con = self.collections[icon]
-            trans = con.get_transform()
+            trans = self.get_transform()
             lw = self._get_nth_label_width(idx)
             additions = []
             paths = con.get_paths()
@@ -627,7 +628,7 @@ layers : array
 
 
 @_docstring.dedent_interpd
-class ContourSet(cm.ScalarMappable, ContourLabeler):
+class ContourSet(mcoll.Collection, ContourLabeler):
     """
     Store a set of contour lines or filled regions.
 
@@ -721,23 +722,28 @@ class ContourSet(cm.ScalarMappable, ContourLabeler):
             Keyword arguments are as described in the docstring of
             `~.Axes.contour`.
         """
+        if antialiased is None and filled:
+            # Eliminate artifacts; we are not stroking the boundaries.
+            antialiased = False
+            # The default for line contours will be taken from the
+            # LineCollection default, which uses :rc:`lines.antialiased`.
+        super().__init__(
+            antialiaseds=antialiased,
+            alpha=alpha,
+            transform=transform,
+        )
         self.axes = ax
         self.levels = levels
         self.filled = filled
-        self.linewidths = linewidths
-        self.linestyles = linestyles
+        # FIXME: We actually need a new draw() method which detects whether
+        # hatches is set (for filled contours) and, if so, performs the right
+        # decomposition to perform the draw as a series of draw_path (with
+        # hatching) instead of the single call to draw_path_collection.
         self.hatches = hatches
-        self.alpha = alpha
         self.origin = origin
         self.extent = extent
         self.colors = colors
         self.extend = extend
-        self.antialiased = antialiased
-        if self.antialiased is None and self.filled:
-            # Eliminate artifacts; we are not stroking the boundaries.
-            self.antialiased = False
-            # The default for line contours will be taken from the
-            # LineCollection default, which uses :rc:`lines.antialiased`.
 
         self.nchunk = nchunk
         self.locator = locator
@@ -758,8 +764,6 @@ class ContourSet(cm.ScalarMappable, ContourLabeler):
         if self.origin == 'image':
             self.origin = mpl.rcParams['image.origin']
 
-        self._transform = transform
-
         self.negative_linestyles = negative_linestyles
         # If negative_linestyles was not defined as a keyword argument, define
         # negative_linestyles with rcParams
@@ -803,85 +807,50 @@ class ContourSet(cm.ScalarMappable, ContourLabeler):
                 if self._extend_max:
                     cmap.set_over(self.colors[-1])
 
-        self.collections = cbook.silent_list(None)
-
         # label lists must be initialized here
         self.labelTexts = []
         self.labelCValues = []
 
-        kw = {'cmap': cmap}
+        self.set_cmap(cmap)
         if norm is not None:
-            kw['norm'] = norm
-        # sets self.cmap, norm if needed;
-        cm.ScalarMappable.__init__(self, **kw)
+            self.set_norm(norm)
         if vmin is not None:
             self.norm.vmin = vmin
         if vmax is not None:
             self.norm.vmax = vmax
         self._process_colors()
 
-        if getattr(self, 'allsegs', None) is None:
-            self.allsegs, self.allkinds = self._get_allsegs_and_allkinds()
-        elif self.allkinds is None:
-            # allsegs specified in constructor may or may not have allkinds as
-            # well.  Must ensure allkinds can be zipped below.
-            self.allkinds = [None] * len(self.allsegs)
-
-        # Each entry in (allsegs, allkinds) is a list of (segs, kinds) which
-        # specifies a list of Paths; but kinds can be None too in which case
-        # all paths in that list are codeless.
-        allpaths = [
-            [*map(mpath.Path,
-                  segs,
-                  kinds if kinds is not None else itertools.repeat(None))]
-            for segs, kinds in zip(self.allsegs, self.allkinds)]
+        if self._paths is None:
+            self._paths = self._make_paths_from_contour_generator()
 
         if self.filled:
-            if self.linewidths is not None:
+            if linewidths is not None:
                 _api.warn_external('linewidths is ignored by contourf')
             # Lower and upper contour levels.
             lowers, uppers = self._get_lowers_and_uppers()
             # Default zorder taken from Collection
             self._contour_zorder = kwargs.pop('zorder', 1)
+            self.set(
+                edgecolor="none",
+                zorder=self._contour_zorder,
+            )
 
-            self.collections[:] = [
-                mcoll.PathCollection(
-                    paths,
-                    antialiaseds=(self.antialiased,),
-                    edgecolors='none',
-                    alpha=self.alpha,
-                    transform=self.get_transform(),
-                    zorder=self._contour_zorder)
-                for level, level_upper, paths
-                in zip(lowers, uppers, allpaths)]
         else:
-            self.tlinewidths = tlinewidths = self._process_linewidths()
-            tlinestyles = self._process_linestyles()
-            aa = self.antialiased
-            if aa is not None:
-                aa = (self.antialiased,)
             # Default zorder taken from LineCollection, which is higher than
             # for filled contours so that lines are displayed on top.
             self._contour_zorder = kwargs.pop('zorder', 2)
+            self.set(
+                facecolor="none",
+                linewidths=self._process_linewidths(linewidths),
+                linestyle=self._process_linestyles(linestyles),
+                zorder=self._contour_zorder,
+                label="_nolegend_",
+            )
 
-            self.collections[:] = [
-                mcoll.PathCollection(
-                    paths,
-                    facecolors="none",
-                    antialiaseds=aa,
-                    linewidths=width,
-                    linestyles=[lstyle],
-                    alpha=self.alpha,
-                    transform=self.get_transform(),
-                    zorder=self._contour_zorder,
-                    label='_nolegend_')
-                for level, width, lstyle, paths
-                in zip(self.levels, tlinewidths, tlinestyles, allpaths)]
 
-        for col in self.collections:
-            self.axes.add_collection(col, autolim=False)
-            col.sticky_edges.x[:] = [self._mins[0], self._maxs[0]]
-            col.sticky_edges.y[:] = [self._mins[1], self._maxs[1]]
+        self.axes.add_collection(self, autolim=False)
+        self.sticky_edges.x[:] = [self._mins[0], self._maxs[0]]
+        self.sticky_edges.y[:] = [self._mins[1], self._maxs[1]]
         self.axes.update_datalim([self._mins, self._maxs])
         self.axes.autoscale_view(tight=True)
 
@@ -893,6 +862,15 @@ class ContourSet(cm.ScalarMappable, ContourLabeler):
                 ", ".join(map(repr, kwargs))
             )
 
+    allsegs = _api.deprecated("3.8")(property(lambda self: [
+        p.vertices for c in self.collections for p in c.get_paths()]))  # FIXME
+    allkinds = _api.deprecated("3.8")(property(lambda self: [
+        p.codes for c in self.collections for p in c.get_paths()]))  # FIXME
+
+    # FIXME: Provide backcompat aliases.
+    tlinewidths = ...
+    tcolors = ...
+
     def get_transform(self):
         """Return the `.Transform` instance used by this ContourSet."""
         if self._transform is None:
@@ -935,9 +913,10 @@ class ContourSet(cm.ScalarMappable, ContourLabeler):
         artists = []
         labels = []
 
+        # FIXME: Adapt to new API.
         if self.filled:
             lowers, uppers = self._get_lowers_and_uppers()
-            n_levels = len(self.collections)
+            n_levels = len(self._path_collection.get_paths())
 
             for i, (collection, lower, upper) in enumerate(
                     zip(self.collections, lowers, uppers)):
@@ -977,51 +956,63 @@ class ContourSet(cm.ScalarMappable, ContourLabeler):
         Must set self.levels, self.zmin and self.zmax, and update axes limits.
         """
         self.levels = args[0]
-        self.allsegs = args[1]
-        self.allkinds = args[2] if len(args) > 2 else None
+        allsegs = args[1]
+        allkinds = args[2] if len(args) > 2 else None
         self.zmax = np.max(self.levels)
         self.zmin = np.min(self.levels)
 
+        if allkinds is None:
+            allkinds = [[None] * len(segs) for segs in allsegs]
+
         # Check lengths of levels and allsegs.
         if self.filled:
-            if len(self.allsegs) != len(self.levels) - 1:
+            if len(allsegs) != len(self.levels) - 1:
                 raise ValueError('must be one less number of segments as '
                                  'levels')
         else:
-            if len(self.allsegs) != len(self.levels):
+            if len(allsegs) != len(self.levels):
                 raise ValueError('must be same number of segments as levels')
 
         # Check length of allkinds.
-        if (self.allkinds is not None and
-                len(self.allkinds) != len(self.allsegs)):
+        if len(allkinds) != len(allsegs):
             raise ValueError('allkinds has different length to allsegs')
 
         # Determine x, y bounds and update axes data limits.
-        flatseglist = [s for seg in self.allsegs for s in seg]
+        flatseglist = [s for seg in allsegs for s in seg]
         points = np.concatenate(flatseglist, axis=0)
         self._mins = points.min(axis=0)
         self._maxs = points.max(axis=0)
 
+        # Each entry in (allsegs, allkinds) is a list of (segs, kinds) which
+        # gets concatenated to construct a Path.
+        self._paths = [
+            mpath.Path.make_compound_path(*map(mpath.Path, segs, kinds))
+            for segs, kinds in zip(allsegs, allkinds)]
+
         return kwargs
 
-    def _get_allsegs_and_allkinds(self):
-        """Compute ``allsegs`` and ``allkinds`` using C extension."""
-        allsegs = []
-        allkinds = []
+    def _make_paths_from_contour_generator(self):
+        """Compute ``paths`` using C extension."""
+        if self._paths is not None:
+            return self._paths
+        paths = []
+        empty_path = mpath.Path(np.empty((0, 2)))
         if self.filled:
             lowers, uppers = self._get_lowers_and_uppers()
             for level, level_upper in zip(lowers, uppers):
                 vertices, kinds = \
                     self._contour_generator.create_filled_contour(
                         level, level_upper)
-                allsegs.append(vertices)
-                allkinds.append(kinds)
+                paths.append(
+                    mpath.Path(np.concatenate(vertices), np.concatenate(kinds))
+                    if len(vertices) else empty_path)
         else:
             for level in self.levels:
                 vertices, kinds = self._contour_generator.create_contour(level)
-                allsegs.append(vertices)
-                allkinds.append(kinds)
-        return allsegs, allkinds
+                paths.append(
+                    mpath.Path(np.concatenate(vertices), np.concatenate(kinds))
+                    if len(vertices) else empty_path)
+        return paths
 
     def _get_lowers_and_uppers(self):
         """
@@ -1048,18 +1039,11 @@ class ContourSet(cm.ScalarMappable, ContourLabeler):
         # so if vmin/vmax are not set yet, this would override them with
         # content from *cvalues* rather than levels like we want
         self.norm.autoscale_None(self.levels)
-        tcolors = [(tuple(rgba),)
-                   for rgba in self.to_rgba(self.cvalues, alpha=self.alpha)]
-        self.tcolors = tcolors
-        hatches = self.hatches * len(tcolors)
-        for color, hatch, collection in zip(tcolors, hatches,
-                                            self.collections):
-            if self.filled:
-                collection.set_facecolor(color)
-                # update the collection's hatch (may be None)
-                collection.set_hatch(hatch)
-            else:
-                collection.set_edgecolor(color)
+        tcolors = self.to_rgba(self.cvalues, alpha=self.alpha)
+        if self.filled:
+            self.set_facecolors(tcolors)
+        else:
+            self.set_edgecolors(tcolors)
         for label, cv in zip(self.labelTexts, self.labelCValues):
             label.set_alpha(self.alpha)
             label.set_color(self.labelMappable.to_rgba(cv))
@@ -1214,16 +1198,13 @@ class ContourSet(cm.ScalarMappable, ContourLabeler):
         if self.extend in ('both', 'max', 'min'):
             self.norm.clip = False
 
-        # self.tcolors are set by the "changed" method
-
-    def _process_linewidths(self):
-        linewidths = self.linewidths
+    def _process_linewidths(self, linewidths):
         Nlev = len(self.levels)
         if linewidths is None:
             default_linewidth = mpl.rcParams['contour.linewidth']
             if default_linewidth is None:
                 default_linewidth = mpl.rcParams['lines.linewidth']
-            tlinewidths = [(default_linewidth,)] * Nlev
+            tlinewidths = [default_linewidth] * Nlev
         else:
             if not np.iterable(linewidths):
                 linewidths = [linewidths] * Nlev
@@ -1234,11 +1215,10 @@ class ContourSet(cm.ScalarMappable, ContourLabeler):
                     linewidths = linewidths * nreps
                 if len(linewidths) > Nlev:
                     linewidths = linewidths[:Nlev]
-            tlinewidths = [(w,) for w in linewidths]
+            tlinewidths = [w for w in linewidths]
         return tlinewidths
 
-    def _process_linestyles(self):
-        linestyles = self.linestyles
+    def _process_linestyles(self, linestyles):
         Nlev = len(self.levels)
         if linestyles is None:
             tlinestyles = ['solid'] * Nlev
@@ -1319,7 +1299,7 @@ class ContourSet(cm.ScalarMappable, ContourLabeler):
             raise ValueError("Method does not support filled contours.")
 
         if indices is None:
-            indices = range(len(self.collections))
+            indices = range(len(self._paths))
 
         d2min = np.inf
         conmin = None
@@ -1330,6 +1310,7 @@ class ContourSet(cm.ScalarMappable, ContourLabeler):
 
         point = np.array([x, y])
 
+        # FIXME: Adapt to new API.
         for icon in indices:
             con = self.collections[icon]
             trans = con.get_transform()
@@ -1352,11 +1333,6 @@ class ContourSet(cm.ScalarMappable, ContourLabeler):
 
         return (conmin, segmin, imin, xmin, ymin, d2min)
 
-    def remove(self):
-        super().remove()
-        for coll in self.collections:
-            coll.remove()
-
 
 @_docstring.dedent_interpd
 class QuadContourSet(ContourSet):