Always attach a manager attribute (possibly None) on canvas.

anntzer · anntzer · commit 3ccc17b739b3 · 2019-09-25T18:02:10.000+02:00
This avoids the confusion in the earlier implementation of Figure.show,
which checked separately for whether the attribute existed and whether
it is None, and (wrongly, I believe) handled the two cases differently.
diff --git a/doc/api/next_api_changes/2018-09-25-AL.rst b/doc/api/next_api_changes/2018-09-25-AL.rst
@@ -0,0 +1,7 @@
+``FigureCanvasBase`` now always has a ``manager`` attribute, which may be None
+``````````````````````````````````````````````````````````````````````````````
+
+Previously, it did not necessarily have such an attribute.  A check for
+``hasattr(figure.canvas, "manager")`` should now be replaced by
+``figure.canvas.manager is not None`` (or ``getattr(figure.canvas, "manager", None) is not None``
+for back-compatibility).
diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
@@ -1627,6 +1627,7 @@ def __init__(self, figure):
         self._is_saving = False
         figure.set_canvas(self)
         self.figure = figure
+        self.manager = None
         # a dictionary from event name to a dictionary that maps cid->func
         self.callbacks = cbook.CallbackRegistry()
         self.widgetlock = widgets.LockDraw()
@@ -2127,7 +2128,7 @@ def get_window_title(self):
         Get the title text of the window containing the figure.
         Return None if there is no window (e.g., a PS backend).
         """
-        if hasattr(self, "manager"):
+        if self.manager:
             return self.manager.get_window_title()
 
     def set_window_title(self, title):
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -430,25 +430,18 @@ def show(self, warn=True):
             If ``True`` and we are not running headless (i.e. on Linux with an
             unset DISPLAY), issue warning when called on a non-GUI backend.
         """
+        if self.canvas.manager is None:
+            raise AttributeError(
+                "Figure.show works only for figures managed by pyplot, "
+                "normally created by pyplot.figure()")
         try:
-            manager = getattr(self.canvas, 'manager')
-        except AttributeError as err:
-            raise AttributeError("%s\n"
-                                 "Figure.show works only "
-                                 "for figures managed by pyplot, normally "
-                                 "created by pyplot.figure()." % err)
-
-        if manager is not None:
-            try:
-                manager.show()
-                return
-            except NonGuiException:
-                pass
-        if (backends._get_running_interactive_framework() != "headless"
-                and warn):
-            cbook._warn_external('Matplotlib is currently using %s, which is '
-                                 'a non-GUI backend, so cannot show the '
-                                 'figure.' % get_backend())
+            self.canvas.manager.show()
+        except NonGuiException:
+            if (backends._get_running_interactive_framework() != "headless"
+                    and warn):
+                cbook._warn_external(
+                    f"Matplotlib is currently using {get_backend()}, which is "
+                    f"a non-GUI backend, so cannot show the figure.")
 
     def _get_axes(self):
         return self._axstack.as_list()
