Consistent handling of *args in Axes.stem

Zac-HD · Zac-HD · commit ad389f6f2bcf · 2018-05-23T11:42:37.000+10:00
diff --git a/doc/api/next_api_changes/2018-05-06-ZHD.rst b/doc/api/next_api_changes/2018-05-06-ZHD.rst
@@ -0,0 +1,7 @@
+Consistent handling of \*args in Axes.stem
+------------------------------------------
+
+:meth:`matplotlib.axex.Axes.stem` now raises TypeError when passed
+unhandled positional arguments.  If two or more arguments are passed
+(ie X, Y, [linefmt], ...) and Y cannot be cast to an array, an error
+will be raised instead of treating X as Y and Y as linefmt.
diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
@@ -2350,17 +2350,19 @@ def stem(self, *args, linefmt=None, markerfmt=None, basefmt=None,
             which inspired this method.
 
         """
-        # Assume there's at least one data array
+        if not 1 <= len(args) <= 5:
+            raise TypeError('stem expected between 1 and 5 positional '
+                            'arguments, got {}'.format(args))
+
         y = np.asarray(args[0])
         args = args[1:]
 
         # Try a second one
-        try:
-            x, y = y, np.asarray(args[0], dtype=float)
-        except (IndexError, ValueError):
-            # The second array doesn't make sense, or it doesn't exist
+        if not args:
             x = np.arange(len(y))
         else:
+            x = y
+            y = np.asarray(args[0], dtype=float)
             args = args[1:]
 
         # defaults for formats
