MNT: Remove deprecated axes kwargs collision detection

In Matplotlib 2.1, the behavior of reusing existing axes when
created with the same arguments was deprecated (see matplotlib#9037). This
behavior is now removed.

The behavior of the functions to create new axes (`pyplot.axes`,
`pyplot.subplot`, `figure.Figure.add_axes`,
`figure.Figure.add_subplot`) has changed. In the past, these
functions would detect if you were attempting to create Axes with
the same keyword arguments as already-existing axes in the current
figure, and if so, they would return the existing Axes. Now, these
functions will always create new Axes. A special exception is
`pyplot.subplot`, which will reuse any existing subplot with a
matching subplot spec. However, if there is a subplot with a
matching subplot spec, then that subplot will be returned, even if
the keyword arguments with which it was created differ.

Correspondingly, the behavior of the functions to get the current
Axes (`pyplot.gca`, `figure.Figure.gca`) has changed. In the past,
these functions accepted keyword arguments. If the keyword
arguments matched an already-existing Axes, then that Axes would
be returned, otherwise new Axes would be created with those
keyword arguments. Now, the keyword arguments are only considered
if there are no axes at all in the current figure. In a future
release, these functions will not accept keyword arguments at all.

Fixes matplotlib#18832.

Author: lpsinger

doc/api/next_api_changes/deprecations/19153-LPS.rst

@@ -0,0 +1,5 @@
+``pyplot.gca()``, ``Figure.gca``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Passing keyword arguments to `.pyplot.gca` or `.figure.Figure.gca` will not be
+supported in a future release.

doc/users/next_whats_new/axes_kwargs_collision.rst

@@ -0,0 +1,21 @@
+Changes to behavior of Axes creation methods (``gca()``, ``add_axes()``, ``add_subplot()``)
+-------------------------------------------------------------------------------------------
+
+The behavior of the functions to create new axes (`.pyplot.axes`,
+`.pyplot.subplot`, `.figure.Figure.add_axes`,
+`.figure.Figure.add_subplot`) has changed. In the past, these functions would
+detect if you were attempting to create Axes with the same keyword arguments as
+already-existing axes in the current figure, and if so, they would return the
+existing Axes. Now, these functions will always create new Axes. A special
+exception is `.pyplot.subplot`, which will reuse any existing subplot with a
+matching subplot spec. However, if there is a subplot with a matching subplot
+spec, then that subplot will be returned, even if the keyword arguments with
+which it was created differ.
+
+Correspondingly, the behavior of the functions to get the current Axes
+(`.pyplot.gca`, `.figure.Figure.gca`) has changed. In the past, these functions
+accepted keyword arguments. If the keyword arguments matched an
+already-existing Axes, then that Axes would be returned, otherwise new Axes
+would be created with those keyword arguments. Now, the keyword arguments are
+only considered if there are no axes at all in the current figure. In a future
+release, these functions will not accept keyword arguments at all.

lib/matplotlib/axes/_subplots.py

@@ -1,5 +1,4 @@
 import functools
-import uuid
 
 from matplotlib import _api, docstring
 import matplotlib.artist as martist
@@ -142,16 +141,7 @@ def _make_twin_axes(self, *args, **kwargs):
             # which currently uses this internal API.
             if kwargs["sharex"] is not self and kwargs["sharey"] is not self:
                 raise ValueError("Twinned Axes may share only one axis")
-        # The dance here with label is to force add_subplot() to create a new
-        # Axes (by passing in a label never seen before).  Note that this does
-        # not affect plot reactivation by subplot() as twin axes can never be
-        # reactivated by subplot().
-        sentinel = str(uuid.uuid4())
-        real_label = kwargs.pop("label", sentinel)
-        twin = self.figure.add_subplot(
-            self.get_subplotspec(), *args, label=sentinel, **kwargs)
-        if real_label is not sentinel:
-            twin.set_label(real_label)
+        twin = self.figure.add_subplot(self.get_subplotspec(), *args, **kwargs)
         self.set_adjustable('datalim')
         twin.set_adjustable('datalim')
         self._twinned_axes.join(self, twin)

lib/matplotlib/cbook/__init__.py

@@ -606,6 +606,9 @@ def __len__(self):
     def __getitem__(self, ind):
         return self._elements[ind]
 
+    def as_list(self):
+        return list(self._elements)
+
     def forward(self):
         """Move the position forward and return the current element."""
         self._pos = min(self._pos + 1, len(self._elements) - 1)