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

anntzer · anntzer · commit adfb193d7908 · 2018-08-21T10:13:50.000+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
@@ -137,7 +137,7 @@
 
 # cbook must import matplotlib only within function
 # definitions, so it is safe to import from it here.
-from . import cbook
+from . import cbook, rcsetup
 from matplotlib.cbook import (
     MatplotlibDeprecationWarning, dedent, get_label, sanitize_sequence)
 from matplotlib.cbook import mplDeprecation  # deprecated
@@ -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):
@@ -1357,7 +1363,7 @@ def use(arg, warn=True, force=False):
 
 
 if os.environ.get('MPLBACKEND'):
-    use(os.environ['MPLBACKEND'])
+    rcParams['backend'] = os.environ.get('MPLBACKEND')
 
 
 def get_backend():
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()
diff --git a/lib/matplotlib/testing/__init__.py b/lib/matplotlib/testing/__init__.py
@@ -36,7 +36,7 @@ def setup():
                 "Could not set locale to English/United States. "
                 "Some date-related tests may fail.")
 
-    mpl.use('Agg', warn=False)  # use Agg backend for these tests
+    mpl.use('Agg', force=True, warn=False)  # use Agg backend for these tests
 
     with warnings.catch_warnings():
         warnings.simplefilter("ignore", MatplotlibDeprecationWarning)
diff --git a/lib/matplotlib/testing/conftest.py b/lib/matplotlib/testing/conftest.py
@@ -8,7 +8,7 @@
 
 
 def pytest_configure(config):
-    matplotlib.use('agg')
+    matplotlib.use('agg', force=True)
     matplotlib._called_from_pytest = True
     matplotlib._init_tests()
 
