Fix polar() regression on second call failure

Author: beckja

lib/matplotlib/pyplot.py

@@ -2193,14 +2193,12 @@ def polar(*args, **kwargs):
     # If an axis already exists, check if it has a polar projection
     if gcf().get_axes():
         ax = gca()
-        if isinstance(ax, PolarAxes):
-            return ax
-        else:
+        if not isinstance(ax, PolarAxes):
             _api.warn_external('Trying to create polar plot on an Axes '
                                'that does not have a polar projection.')
-    ax = axes(projection="polar")
-    ret = ax.plot(*args, **kwargs)
-    return ret
+    else:
+        ax = axes(projection="polar")
+    return ax.plot(*args, **kwargs)
 
 
 # If rcParams['backend_fallback'] is true, and an interactive backend is

lib/matplotlib/tests/test_pyplot.py

@@ -310,3 +310,13 @@ def test_subplot_change_projection():
         assert ax_next.name == proj
         assert ax is not ax_next
         ax = ax_next
+
+
+def test_polar_second_call():
+    # the first call creates the axes with polar projection
+    h1 = plt.polar(0., 1., 'ro')
+    assert isinstance(h1[0], mpl.lines.Line2D)
+    # the second call should reuse the existing axes
+    h2 = plt.polar(1.57, .5, 'bo')
+    assert isinstance(h2[0], mpl.lines.Line2D)
+    assert h1[0].axes is h2[0].axes