Simplify _process_plot_var_args.set_prop_cycle.

It is private, so we can restrict it to only support Cycler instances
and None (rather than also supporting `*args, **kwargs` from which a
new Cycler is constructed: the caller Axes.set_prop_cycle already takes
care of conversion into Cycler instances).

Author: anntzer

lib/matplotlib/axes/_base.py

@@ -223,26 +223,21 @@ class _process_plot_var_args:
     def __init__(self, axes, command='plot'):
         self.axes = axes
         self.command = command
-        self.set_prop_cycle()
+        self.set_prop_cycle(None)
 
     def __getstate__(self):
         # note: it is not possible to pickle a generator (and thus a cycler).
         return {'axes': self.axes, 'command': self.command}
 
     def __setstate__(self, state):
         self.__dict__ = state.copy()
-        self.set_prop_cycle()
+        self.set_prop_cycle(None)
 
-    def set_prop_cycle(self, *args, **kwargs):
-        # Can't do `args == (None,)` as that crashes cycler.
-        if not (args or kwargs) or (len(args) == 1 and args[0] is None):
-            prop_cycler = mpl.rcParams['axes.prop_cycle']
-        else:
-            prop_cycler = cycler(*args, **kwargs)
-
-        self.prop_cycler = itertools.cycle(prop_cycler)
-        # This should make a copy
-        self._prop_keys = prop_cycler.keys
+    def set_prop_cycle(self, cycler):
+        if cycler is None:
+            cycler = mpl.rcParams['axes.prop_cycle']
+        self.prop_cycler = itertools.cycle(cycler)
+        self._prop_keys = cycler.keys  # This should make a copy
 
     def __call__(self, *args, data=None, **kwargs):
         self.axes._process_unit_info(kwargs=kwargs)