FIX: treat zorder=None as falling back to the default

closes #9785
matplotlib · efiring · Nov 16, 2017 · Nov 16, 2017 · Nov 16, 2017 · ed02d72e92102b92072216cb3f05bc0b9ad4ccad
commit ed02d72e92102b92072216cb3f05bc0b9ad4ccad
diff --git a/lib/matplotlib/artist.py b/lib/matplotlib/artist.py
@@ -885,6 +885,8 @@ def set_zorder(self, level):

        ACCEPTS: any number
        """
+        if level is None:
+            level = self.__class__.zorder
        self.zorder = level
        self.pchanged()
        self.stale = True

diff --git a/lib/matplotlib/tests/test_artist.py b/lib/matplotlib/tests/test_artist.py
@@ -246,3 +246,13 @@ def test_setp():
    sio = io.StringIO()
    plt.setp(lines1, 'zorder', file=sio)
    assert sio.getvalue() == '  zorder: any number \n'
+
+
+def test_None_zorder():
+    fig, ax = plt.subplots()
+    ln, = ax.plot(range(5), zorder=None)
+    assert ln.get_zorder() == mlines.Line2D.zorder
+    ln.set_zorder(123456)
+    assert ln.get_zorder() == 123456
+    ln.set_zorder(None)
+    assert ln.get_zorder() == mlines.Line2D.zorder