Move non-gui warning message to backend_bases.

anntzer · anntzer · commit 154a6161ab2c · 2019-09-26T01:42:02.000+02:00
This avoids 1) splitting the emission of the NonGuiException and the
generation of the actual message in two places, and 2) allows replacing
the rather roundabout `manager.canvas.figure.show()` by just
`manager.show()` in `plt.show()`  (previously, the two only differed by
the fact that the former would convert the NonGuiException thrown by the
latter into an exception.
diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
@@ -2563,11 +2563,15 @@ def notify_axes_change(fig):
     def show(self):
         """
         For GUI backends, show the figure window and redraw.
-        For non-GUI backends, raise an exception to be caught
-        by :meth:`~matplotlib.figure.Figure.show`, for an
-        optional warning.
+        For non-GUI backends, raise an exception, unless running headless (i.e.
+        on Linux with an unset DISPLAY); this exception is converted to a
+        warning in `.Figure.show`.
         """
-        raise NonGuiException()
+        # This should be overridden in GUI backends.
+        if mpl.backends._get_running_interactive_framework() != "headless":
+            raise NonGuiException(
+                f"Matplotlib is currently using {get_backend()}, which is "
+                f"a non-GUI backend, so cannot show the figure.")
 
     def destroy(self):
         pass
@@ -3360,8 +3364,10 @@ def show(cls, block=None):
         if not managers:
             return
         for manager in managers:
-            # Emits a warning if the backend is non-interactive.
-            manager.canvas.figure.show()
+            try:
+                manager.show()  # Emits a warning for non-interactive backend.
+            except NonGuiException as exc:
+                cbook._warn_external(str(exc))
         if cls.mainloop is None:
             return
         if block is None:
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -436,12 +436,8 @@ def show(self, warn=True):
                 "normally created by pyplot.figure()")
         try:
             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.")
+        except NonGuiException as exc:
+            cbook._warn_external(str(exc))
 
     def _get_axes(self):
         return self._axstack.as_list()
