Add deprecation for setting data with non sequence type in Line2D

doc/api/next_api_changes/deprecations/25196-EP.rst

@@ -0,0 +1,4 @@
+``Line2D``
+~~~~~~~~~~
+When creating a Line2D or using `.Line2D.set_xdata` and `.Line2D.set_ydata`,
+passing x/y data as non sequence is deprecated.

lib/matplotlib/lines.py

@@ -1274,7 +1274,14 @@ def set_xdata(self, x):
         x : 1D array
         """
         if not np.iterable(x):
-            raise RuntimeError('x must be a sequence')
+            # When deprecation cycle is completed
+            # raise RuntimeError('x must be a sequence')
+            _api.warn_deprecated(
+                since=3.7,
+                message="Setting data with a non sequence type "
+                "is deprecated since %(since)s and will be "
+                "remove %(removal)s")
+            x = [x, ]
         self._xorig = copy.copy(x)
         self._invalidx = True
         self.stale = True
@@ -1288,7 +1295,14 @@ def set_ydata(self, y):
         y : 1D array
         """
         if not np.iterable(y):
-            raise RuntimeError('y must be a sequence')
+            # When deprecation cycle is completed
+            # raise RuntimeError('y must be a sequence')
+            _api.warn_deprecated(
+                since=3.7,
+                message="Setting data with a non sequence type "
+                "is deprecated since %(since)s and will be "
+                "remove %(removal)s")
+            y = [y, ]
         self._yorig = copy.copy(y)
         self._invalidy = True
         self.stale = True

lib/matplotlib/tests/test_lines.py

@@ -20,6 +20,7 @@
 import matplotlib.pyplot as plt
 import matplotlib.transforms as mtransforms
 from matplotlib.testing.decorators import image_comparison, check_figures_equal
+from matplotlib._api.deprecation import MatplotlibDeprecationWarning
 
 
 def test_segment_hits():
@@ -91,9 +92,12 @@ def test_invalid_line_data():
         mlines.Line2D([], 1)
 
     line = mlines.Line2D([], [])
-    with pytest.raises(RuntimeError, match='x must be'):
+    # when deprecation cycle is completed
+    # with pytest.raises(RuntimeError, match='x must be'):
+    with pytest.warns(MatplotlibDeprecationWarning):
         line.set_xdata(0)
-    with pytest.raises(RuntimeError, match='y must be'):
+    # with pytest.raises(RuntimeError, match='y must be'):
+    with pytest.warns(MatplotlibDeprecationWarning):
         line.set_ydata(0)