MNT: compress swap to one line

Author: tacaswell

lib/matplotlib/axes/_base.py

@@ -1315,8 +1315,7 @@ def __clear(self):
         self._get_patches_for_fill = _process_plot_var_args(self, 'fill')
 
         self._gridOn = mpl.rcParams['axes.grid']
-        old_children = getattr(self, '_children', [])
-        self._children = []
+        old_children, self._children = self._children, []
         for chld in old_children:
             chld.axes = chld.figure = None
         self._mouseover_set = _OrderedSet()