Warn on redundant definition of plot properties

`plt.plot(x, y, fmt)` allows to specify marker, linestyle and color via
`fmt`. Warn if there are additionally keyword arguments that specify the
same properties.

Closes matplotlib#19275.

Author: timhoffm

examples/misc/svg_filter_line.py

@@ -20,7 +20,7 @@
 l1, = ax.plot([0.1, 0.5, 0.9], [0.1, 0.9, 0.5], "bo-",
               mec="b", lw=5, ms=10, label="Line 1")
 l2, = ax.plot([0.1, 0.5, 0.9], [0.5, 0.2, 0.7], "rs-",
-              mec="r", lw=5, ms=10, color="r", label="Line 2")
+              mec="r", lw=5, ms=10, label="Line 2")
 
 
 for l in [l1, l2]:

lib/matplotlib/axes/_base.py

@@ -117,6 +117,9 @@ def __call__(self, ax, renderer):
             self._transform - ax.figure.transSubfigure)
 
 
+_FORMAT_UNSET = 'None'
+
+
 def _process_plot_format(fmt):
     """
     Convert a MATLAB style color/line style format string to a (*linestyle*,
@@ -129,6 +132,10 @@ def _process_plot_format(fmt):
     * 'r--': red dashed lines
     * 'C2--': the third color in the color cycle, dashed lines
 
+    The format is absolute in the sense that if a linestyle or marker is not
+    defined in *fmt*, there is no line or marker. This is expressed by
+    returning _FORMAT_UNSET for the respective quantity.
+
     See Also
     --------
     matplotlib.Line2D.lineStyles, matplotlib.colors.cnames
@@ -196,9 +203,9 @@ def _process_plot_format(fmt):
     if linestyle is None and marker is None:
         linestyle = mpl.rcParams['lines.linestyle']
     if linestyle is None:
-        linestyle = 'None'
+        linestyle = _FORMAT_UNSET
     if marker is None:
-        marker = 'None'
+        marker = _FORMAT_UNSET
 
     return linestyle, marker, color
 
@@ -461,6 +468,21 @@ def _plot_args(self, tup, kwargs, return_kwargs=False):
         for prop_name, val in zip(('linestyle', 'marker', 'color'),
                                   (linestyle, marker, color)):
             if val is not None:
+                # check for conflicts between fmt and kwargs
+                if (fmt.lower() != 'none'
+                        and prop_name in kwargs
+                        and val != _FORMAT_UNSET):
+                    # Technically ``plot(x, y, 'o', ls='--')`` is a conflict
+                    # because 'o' implicitly unsets the linestyle
+                    # (linestyle=_FORMAT_UNSET).
+                    # We'll gracefully not warn in this case because an
+                    # explicit set via kwargs can be seen as intention to
+                    # override an implicit unset.
+                    _api.warn_external(
+                        f"{prop_name} is redundantly defined by the "
+                        f"'{prop_name}' keyword argument and the fmt string "
+                        f'"{fmt}" (-> {prop_name}={val!r}). The keyword '
+                        f"argument will take precedence.")
                 kw[prop_name] = val
 
         if len(xy) == 2:

lib/matplotlib/tests/test_axes.py

@@ -624,6 +624,17 @@ def test_fill_units():
     fig.autofmt_xdate()
 
 
+def test_plot_format_kwarg_redundant():
+    with pytest.warns(UserWarning, match="marker .* redundantly defined"):
+        plt.plot([0], [0], 'o', marker='x')
+    with pytest.warns(UserWarning, match="linestyle .* redundantly defined"):
+        plt.plot([0], [0], '-', linestyle='--')
+    with pytest.warns(UserWarning, match="color .* redundantly defined"):
+        plt.plot([0], [0], 'r', color='blue')
+    # smoke-test: should not warn
+    plt.errorbar([0], [0], fmt='none', color='blue')
+
+
 @image_comparison(['single_point', 'single_point'])
 def test_single_point():
     # Issue #1796: don't let lines.marker affect the grid