Rewrite AxesStack independently of cbook.Stack.

AxesStack is fairly independent from cbook.Stack: cbook.Stack handles
the forward/back/home buttons of the navbar, and therefore additionally
maintains a movable "cursor" in the stack; AxesStack, on the other hand,
needs to keep track both of "original" order and of "gca" order.
Rewriting it from scratch, and using "original" order as main storage
order (the "gca" stack being tracked using indices) shortens the
implementation and simplifies it (as there's no more need to figure out
what the super()calls do).

Author: anntzer

lib/matplotlib/figure.py

@@ -16,6 +16,7 @@
 
 from contextlib import ExitStack
 import inspect
+import itertools
 import logging
 from numbers import Integral
 
@@ -48,69 +49,41 @@ def _stale_figure_callback(self, val):
         self.figure.stale = val
 
 
-class _AxesStack(cbook.Stack):
+class _AxesStack:
     """
-    Specialization of Stack, to handle all tracking of Axes in a Figure.
+    Helper class to track axes in a figure.
 
-    This stack stores ``ind, axes`` pairs, where ``ind`` is a serial index
-    tracking the order in which axes were added.
-
-    AxesStack is a callable; calling it returns the current axes.
+    Axes are tracked both in the order in which they have been added
+    (``self._axes`` insertion/iteration order) and in the separate "gca" stack
+    (which is the index to which they map in the ``self._axes`` dict).
     """
 
     def __init__(self):
-        super().__init__()
-        self._ind = 0
+        self._axes = {}  # Mapping of axes to "gca" order.
+        self._counter = itertools.count()
 
     def as_list(self):
-        """
-        Return a list of the Axes instances that have been added to the figure.
-        """
-        return [a for i, a in sorted(self._elements)]
-
-    def _entry_from_axes(self, e):
-        return next(((ind, a) for ind, a in self._elements if a == e), None)
+        """List the axes that have been added to the figure."""
+        return [*self._axes]  # This relies on dict preserving order.
 
     def remove(self, a):
         """Remove the axes from the stack."""
-        super().remove(self._entry_from_axes(a))
+        self._axes.pop(a)
 
     def bubble(self, a):
-        """
-        Move the given axes, which must already exist in the stack, to the top.
-        """
-        return super().bubble(self._entry_from_axes(a))
+        """Move an axes, which must already exist in the stack, to the top."""
+        if a not in self._axes:
+            raise ValueError("Axes has not been added yet")
+        self._axes[a] = next(self._counter)
 
     def add(self, a):
-        """
-        Add Axes *a* to the stack.
-
-        If *a* is already on the stack, don't add it again.
-        """
-        # All the error checking may be unnecessary; but this method
-        # is called so seldom that the overhead is negligible.
-        _api.check_isinstance(Axes, a=a)
-
-        if a in self:
-            return
-
-        self._ind += 1
-        super().push((self._ind, a))
+        """Add an axes to the stack, ignoring it if already present."""
+        if a not in self._axes:
+            self._axes[a] = next(self._counter)
 
-    def __call__(self):
-        """
-        Return the active axes.
-
-        If no axes exists on the stack, then returns None.
-        """
-        if not len(self._elements):
-            return None
-        else:
-            index, axes = self._elements[self._pos]
-            return axes
-
-    def __contains__(self, a):
-        return a in self.as_list()
+    def current(self):
+        """Return the active axes, or None if the stack is empty."""
+        return max(self._axes, key=self._axes.__getitem__, default=None)
 
 
 class SubplotParams:
@@ -1503,10 +1476,8 @@ def gca(self, **kwargs):
                 "new axes with default keyword arguments. To create a new "
                 "axes with non-default arguments, use plt.axes() or "
                 "plt.subplot().")
-        if self._axstack.empty():
-            return self.add_subplot(1, 1, 1, **kwargs)
-        else:
-            return self._axstack()
+        ax = self._axstack.current()
+        return ax if ax is not None else self.add_subplot(**kwargs)
 
     def _gci(self):
         # Helper for `~matplotlib.pyplot.gci`.  Do not use elsewhere.
@@ -1525,13 +1496,13 @@ def _gci(self):
         Historically, the only colorable artists were images; hence the name
         ``gci`` (get current image).
         """
-        # Look first for an image in the current Axes:
-        if self._axstack.empty():
+        # Look first for an image in the current Axes.
+        ax = self._axstack.current()
+        if ax is None:
             return None
-        im = self._axstack()._gci()
+        im = ax._gci()
         if im is not None:
             return im
-
         # If there is no image in the current Axes, search for
         # one in a previously created Axes.  Whether this makes
         # sense is debatable, but it is the documented behavior.
@@ -2759,7 +2730,7 @@ def clf(self, keep_observers=False):
         toolbar = getattr(self.canvas, 'toolbar', None)
         if toolbar is not None:
             toolbar.update()
-        self._axstack.clear()
+        self._axstack = _AxesStack()
         self.artists = []
         self.lines = []
         self.patches = []