Deprecate MovieWriter.{exec,args}_key, making them private.

Builtin writer subclasses additionally define the _exec_key and
_args_key attributes, which indicate the rcParams entries where the
path to the executable and additional command-line arguments to the
executable are stored.  Third-party writers cannot meaningfully set
these as they cannot extend rcParams with new keys.

Hence they are really an internal way of mapping builtin writers to
rcParams and should not be exposed.

Author: anntzer

doc/api/next_api_changes/deprecations.rst

@@ -106,3 +106,7 @@ Likewise, passing the *alpha*, *boundaries*, *values*, *extend*, or *filled*
 parameters with a `.ContourSet` mappable, or the *alpha* parameter with an
 `.Artist` mappable, is deprecated, as the mappable would likewise override
 them.
+
+``args_key`` and ``exec_key`` attributes of builtin `.MovieWriter`\s
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+These attributes are deprecated.

lib/matplotlib/animation.py

@@ -267,9 +267,24 @@ class MovieWriter(AbstractMovieWriter):
     fig : `~matplotlib.figure.Figure`
         The figure to capture data from.
         This must be provided by the sub-classes.
-
     """
 
+    # Builtin writer subclasses additionally define the _exec_key and _args_key
+    # attributes, which indicate the rcParams entries where the path to the
+    # executable and additional command-line arguments to the executable are
+    # stored.  Third-party writers cannot meaningfully set these as they cannot
+    # extend rcParams with new keys.
+
+    @cbook.deprecated("3.3")
+    @property
+    def exec_key(self):
+        return self._exec_key
+
+    @cbook.deprecated("3.3")
+    @property
+    def args_key(self):
+        return self._args_key
+
     def __init__(self, fps=5, codec=None, bitrate=None, extra_args=None,
                  metadata=None):
         """
@@ -409,7 +424,7 @@ def bin_path(cls):
         subclass. This is a class method so that the tool can be looked for
         before making a particular MovieWriter subclass available.
         """
-        return str(mpl.rcParams[cls.exec_key])
+        return str(mpl.rcParams[cls._exec_key])
 
     @classmethod
     def isAvailable(cls):
@@ -573,14 +588,14 @@ class FFMpegBase:
     `MovieWriterBase` sub-class.
     """
 
-    exec_key = 'animation.ffmpeg_path'
-    args_key = 'animation.ffmpeg_args'
+    _exec_key = 'animation.ffmpeg_path'
+    _args_key = 'animation.ffmpeg_args'
 
     @property
     def output_args(self):
         args = ['-vcodec', self.codec]
         extra_args = (self.extra_args if self.extra_args is not None
-                      else mpl.rcParams[self.args_key])
+                      else mpl.rcParams[self._args_key])
         # For h264, the default format is yuv444p, which is not compatible
         # with quicktime (and others). Specifying yuv420p fixes playback on
         # iOS, as well as HTML5 video in firefox and safari (on both Win and
@@ -658,8 +673,8 @@ class AVConvBase(FFMpegBase):
     `MovieWriterBase` sub-class.
     """
 
-    exec_key = 'animation.avconv_path'
-    args_key = 'animation.avconv_args'
+    _exec_key = 'animation.avconv_path'
+    _args_key = 'animation.avconv_args'
 
     # NOTE : should be removed when the same method is removed in FFMpegBase.
     isAvailable = classmethod(MovieWriter.isAvailable.__func__)
@@ -696,8 +711,8 @@ class ImageMagickBase:
     `MovieWriterBase` sub-class.
     """
 
-    exec_key = 'animation.convert_path'
-    args_key = 'animation.convert_args'
+    _exec_key = 'animation.convert_path'
+    _args_key = 'animation.convert_args'
 
     @property
     def delay(self):
@@ -706,7 +721,7 @@ def delay(self):
     @property
     def output_args(self):
         extra_args = (self.extra_args if self.extra_args is not None
-                      else mpl.rcParams[self.args_key])
+                      else mpl.rcParams[self._args_key])
         return [*extra_args, self.outfile]
 
     @classmethod
@@ -784,7 +799,7 @@ def _embedded_frames(frame_list, frame_format):
 @writers.register('html')
 class HTMLWriter(FileMovieWriter):
     supported_formats = ['png', 'jpeg', 'tiff', 'svg']
-    args_key = 'animation.html_args'
+    _args_key = 'animation.html_args'
 
     @classmethod
     def isAvailable(cls):