Resolve backend in rcParams.__getitem__("backend").

anntzer · anntzer · commit 7ca68058a71b · 2018-08-20T09:08:22.000+02:00
... to avoid leaking the non-string sentinel.

Changes to `backends/__init__` and `_backend_selection()` were necessary
to avoid import-time cycles.

The fallback order in `backend_fallback` slightly changed in that the
FooCairo backends now fallback to WxAgg instead of Wx when a wx event
loop is active, but I wouldn't worry too much about it anyways given
that the Wx backend is deprecated and the correct fallback would be
WxCairo to start with.
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -877,6 +877,12 @@ def __getitem__(self, key):
                 "3.0", "{} is deprecated; in the future, examples will be "
                 "found relative to the 'datapath' directory.".format(key))
 
+        elif key == "backend":
+            val = dict.__getitem__(self, key)
+            if val is rcsetup._auto_backend_sentinel:
+                from matplotlib import pyplot as plt
+                plt.switch_backend(rcsetup._auto_backend_sentinel)
+
         return dict.__getitem__(self, key)
 
     def __repr__(self):
diff --git a/lib/matplotlib/backends/__init__.py b/lib/matplotlib/backends/__init__.py
@@ -10,12 +10,9 @@
 
 _log = logging.getLogger(__name__)
 
-backend = matplotlib.get_backend()
-# FIXME: Remove.
-_backend_loading_tb = "".join(
-    line for line in traceback.format_stack()
-    # Filter out line noise from importlib line.
-    if not line.startswith('  File "<frozen importlib._bootstrap'))
+
+# NOTE: plt.switch_backend() (called at import time) will add a "backend"
+# attribute here for backcompat.
 
 
 def _get_running_interactive_framework():
diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
@@ -67,7 +67,7 @@
            Locator, IndexLocator, FixedLocator, NullLocator,\
            LinearLocator, LogLocator, AutoLocator, MultipleLocator,\
            MaxNLocator
-from matplotlib.backends import pylab_setup
+from matplotlib.backends import pylab_setup, _get_running_interactive_framework
 
 _log = logging.getLogger(__name__)
 
@@ -78,35 +78,15 @@
 # FIXME: Deprecate.
 def _backend_selection():
     """
-    If rcParams['backend_fallback'] is true, check to see if the
-    current backend is compatible with the current running event loop,
-    and if not switches to a compatible one.
-    """
-    backend = rcParams['backend']
-    if not rcParams['backend_fallback'] or backend not in _interactive_bk:
-        return
-    is_agg_backend = rcParams['backend'].endswith('Agg')
-    if 'wx' in sys.modules and backend not in ('WX', 'WXAgg'):
-        import wx
-        if wx.App.IsMainLoopRunning():
-            rcParams['backend'] = 'wx' + 'Agg' * is_agg_backend
-    elif 'PyQt4.QtCore' in sys.modules and not backend == 'Qt4Agg':
-        import PyQt4.QtGui
-        if not PyQt4.QtGui.qApp.startingUp():
-            # The mainloop is running.
-            rcParams['backend'] = 'qt4Agg'
-    elif 'PyQt5.QtCore' in sys.modules and not backend == 'Qt5Agg':
-        import PyQt5.QtWidgets
-        if not PyQt5.QtWidgets.qApp.startingUp():
-            # The mainloop is running.
-            rcParams['backend'] = 'qt5Agg'
-    elif 'gtk' in sys.modules and 'gi' in sys.modules:
-        from gi.repository import GLib
-        if GLib.MainLoop().is_running():
-            rcParams['backend'] = 'GTK3Agg'
-    elif 'Tkinter' in sys.modules and not backend == 'TkAgg':
-        # import Tkinter
-        pass  # what if anything do we need to do for tkinter?
+    If rcParams['backend_fallback'] is true, we will check (at backend
+    load-time) to see if the current backend is compatible with the current
+    running event loop, and if not switches to a compatible one.
+    """
+    if rcParams["backend_fallback"]:
+        if (dict.__getitem__(rcParams, "backend") in _interactive_bk
+                and _get_running_interactive_framework()):
+            dict.__setitem__(
+                rcParams, "backend", rcsetup._auto_backend_sentinel)
 
 
 _backend_selection()
