Warn on redundant definition of plot properties

timhoffm · timhoffm · commit 07f7a8351686 · 2021-01-11T21:04:43.000+01:00
`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.
diff --git a/lib/matplotlib/axes/_base.py b/lib/matplotlib/axes/_base.py
@@ -417,10 +417,16 @@ def _plot_args(self, tup, kwargs, return_kwargs=False):
             raise ValueError("x, y, and format string must not be None")
 
         kw = {}
-        for k, v in zip(('linestyle', 'marker', 'color'),
-                        (linestyle, marker, color)):
+        for prop_name, v in zip(('linestyle', 'marker', 'color'),
+                                (linestyle, marker, color)):
             if v is not None:
-                kw[k] = v
+                if prop_name in kwargs:
+                    _api.warn_external(
+                        f"'{prop_name}' is redundantly defined via fmt string "
+                        "and keyword argument. The keyword argument will "
+                        "take precedence."
+                    )
+                kw[prop_name] = v
 
         if len(tup) == 2:
             x = _check_1d(tup[0])
diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
@@ -624,6 +624,15 @@ 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')
+
+
 @image_comparison(['single_point', 'single_point'])
 def test_single_point():
     # Issue #1796: don't let lines.marker affect the grid
