Fix legend dragging.

anntzer · anntzer · commit 12d800d6a25c · 2020-01-05T18:28:18.000+01:00
Without this PR, after
```
plot([1, 2], label="foo"); legend().set_draggable(True)
```
when dragging the legend, releasing the mouse button results in
```
Traceback (most recent call last):
  File ".../matplotlib/cbook/__init__.py", line 236, in process
    func(*args, **kwargs)
  File ".../matplotlib/offsetbox.py", line 1716, in on_release
    self.finalize_offset()
  File ".../matplotlib/legend.py", line 78, in finalize_offset
    {"loc": self._update_loc, "bbox": self._bbox_to_anchor},
AttributeError: 'DraggableLegend' object has no attribute '_bbox_to_anchor'
```
See comment in code for more explanations.

A test may be nice but tricky to write...
diff --git a/lib/matplotlib/legend.py b/lib/matplotlib/legend.py
@@ -74,10 +74,10 @@ def artist_picker(self, legend, evt):
         return self.legend.contains(evt)
 
     def finalize_offset(self):
-        update_method = cbook._check_getitem(
-            {"loc": self._update_loc, "bbox": self._bbox_to_anchor},
-            update=self._update)
-        update_method(self.get_loc_in_canvas())
+        if self._update == "loc":
+            self._update_loc(self.get_loc_in_canvas())
+        elif self._update == "bbox":
+            self._bbox_to_anchor(self.get_loc_in_canvas())
 
     def _update_loc(self, loc_in_canvas):
         bbox = self.legend.get_bbox_to_anchor()
