Make plt.{r,theta}grids act as setters even when all args are kwargs.

I'm going to claim that silently ignoring kwargs was a bug and thus this
behavior can be changed with just a note, but no deprecation.

Author: anntzer

doc/api/next_api_changes/behaviour.rst

@@ -150,3 +150,8 @@ support for it will be dropped in a future Matplotlib release.
 `.font_manager.json_dump` now locks the font manager dump file
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 ... to prevent multiple processes from writing to it at the same time.
+
+`.pyplot.rgrids` and `.pyplot.thetagrids` now act as setters also when called with only kwargs
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Previously, keyword arguments were silently ignored when no positional
+arguments were given.

lib/matplotlib/pyplot.py

@@ -1627,7 +1627,7 @@ def rgrids(*args, **kwargs):
     ax = gca()
     if not isinstance(ax, PolarAxes):
         raise RuntimeError('rgrids only defined for polar axes')
-    if len(args) == 0:
+    if not args and not kwargs:
         lines = ax.yaxis.get_gridlines()
         labels = ax.yaxis.get_ticklabels()
     else:
@@ -1694,7 +1694,7 @@ def thetagrids(*args, **kwargs):
     ax = gca()
     if not isinstance(ax, PolarAxes):
         raise RuntimeError('thetagrids only defined for polar axes')
-    if len(args) == 0:
+    if not args and not kwargs:
         lines = ax.xaxis.get_ticklines()
         labels = ax.xaxis.get_ticklabels()
     else: