Fix edge-case in preprocess_data, if label_namer is optional and unset.

anntzer · anntzer · commit e4458316b763 · 2020-03-01T11:51:18.000+01:00
9728144 removed a call to `BoundArguments.apply_defaults` to guard against an API change in Py3.9. As it turns out this means that `all_kwargs[label_namer]` can now fail, in theory, if _preprocess_data is used on a function for which label_namer is an *optional* parameter (because the default won't be set in all_kwargs). Fortunately, 1) there is no such case (it would be semantically questionable to derive the label from a non-required argument...) and 2) the fix is simple (use `all_kwargs.get` instead -- _label_namer properly handles None).
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -1405,10 +1405,8 @@ def inner(ax, *args, data=None, **kwargs):
 
         if needs_label:
             all_kwargs = {**bound.arguments, **bound.kwargs}
-            # label_namer will be in all_kwargs as we asserted above that
-            # `label_namer is None or label_namer in arg_names`.
             new_kwargs["label"] = _label_from_arg(
-                all_kwargs[label_namer], auto_label)
+                all_kwargs.get(label_namer), auto_label)
 
         return func(*new_args, **new_kwargs)
 
