Deprecate MarkerStyle(None).

See docstring change for the confusion that this resolves.
We can normalize None inputs on the caller side (and note that directly
constructing MarkerStyles is not something that most users need to do).

Author: anntzer

doc/api/next_api_changes/deprecations/21056-AL.rst

@@ -1,3 +1,3 @@
-Calling ``MarkerStyle()`` with no arguments
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Calling ``MarkerStyle()`` with no arguments or ``MarkerStyle(None)``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 ... is deprecated; use ``MarkerStyle("")`` to construct an empty marker style.

lib/matplotlib/lines.py

@@ -368,6 +368,8 @@ def __init__(self, xdata, ydata,
 
         self._color = None
         self.set_color(color)
+        if marker is None:
+            marker = 'none'  # Default.
         self._marker = MarkerStyle(marker, fillstyle)
 
         self._markevery = None

lib/matplotlib/markers.py

@@ -64,9 +64,10 @@
                                       rotated by ``angle``.
 ============================== ====== =========================================
 
-``None`` also means 'nothing' when directly constructing a `.MarkerStyle`, but
-note that there are other contexts where ``marker=None`` instead means "the
-default marker" (e.g. :rc:`scatter.marker` for `.Axes.scatter`).
+As a deprecated feature, ``None`` also means 'nothing' when directly
+constructing a `.MarkerStyle`, but note that there are other contexts where
+``marker=None`` instead means "the default marker" (e.g. :rc:`scatter.marker`
+for `.Axes.scatter`).
 
 Note that special symbols can be defined via the
 :doc:`STIX math font </tutorials/text/mathtext>`,
@@ -202,7 +203,6 @@ class MarkerStyle:
         CARETDOWNBASE: 'caretdownbase',
         "None": 'nothing',
         "none": 'nothing',
-        None: 'nothing',
         ' ': 'nothing',
         '': 'nothing'
     }
@@ -245,6 +245,12 @@ def __init__(self, marker=_unset, fillstyle=None):
                 "deprecated since %(since)s; support will be removed "
                 "%(removal)s.  Use MarkerStyle('') to construct an empty "
                 "MarkerStyle.")
+        if marker is None:
+            marker = ""
+            _api.warn_deprecated(
+                "3.6", message="MarkerStyle(None) is deprecated since "
+                "%(since)s; support will be removed %(removal)s.  Use "
+                "MarkerStyle('') to construct an empty MarkerStyle.")
         self._set_marker(marker)
 
     __init__.__signature__ = inspect.signature(  # Only for deprecation period.

lib/matplotlib/tests/test_axes.py

@@ -4117,9 +4117,15 @@ def test_eventplot_orientation(data, orientation):
 @image_comparison(['marker_styles.png'], remove_text=True)
 def test_marker_styles():
     fig, ax = plt.subplots()
-    for y, marker in enumerate(sorted(
-            {*matplotlib.markers.MarkerStyle.markers} - {"none"},
-            key=lambda x: str(type(x))+str(x))):
+    # Since generation of the test image, None was removed but 'none' was
+    # added. By moving 'none' to the front (=former sorted place of None)
+    # we can avoid regenerating the test image. This can be removed if the
+    # test image has to be regenerated for other reasons.
+    markers = sorted(matplotlib.markers.MarkerStyle.markers,
+                     key=lambda x: str(type(x))+str(x))
+    markers.remove('none')
+    markers = ['none', *markers]
+    for y, marker in enumerate(markers):
         ax.plot((y % 2)*5 + np.arange(10)*10, np.ones(10)*10*y, linestyle='',
                 marker=marker, markersize=10+y/5, label=marker)
 

lib/matplotlib/tests/test_marker.py

@@ -19,7 +19,6 @@ def test_marker_fillstyle():
     'x',
     '',
     'None',
-    None,
     r'$\frac{1}{2}$',
     "$\u266B$",
     1,
@@ -40,10 +39,13 @@ def test_markers_valid(marker):
     markers.MarkerStyle(marker)
 
 
-def test_deprecated_marker_noargs():
+def test_deprecated_marker():
     with pytest.warns(MatplotlibDeprecationWarning):
         ms = markers.MarkerStyle()
     markers.MarkerStyle(ms)  # No warning on copy.
+    with pytest.warns(MatplotlibDeprecationWarning):
+        ms = markers.MarkerStyle(None)
+    markers.MarkerStyle(ms)  # No warning on copy.
 
 
 @pytest.mark.parametrize('marker', [