MNT: special case the handling of 'backend' in rc_context

tacaswell · anntzer · commit 7dffd5bf61af · 2018-08-26T23:00:27.000+02:00
If the context block triggers the auto-resolution of the backend
keep than information around.
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -1292,16 +1292,23 @@ def __init__(self, rc=None, fname=None):
             if rc:
                 rcParams.update(rc)
         except Exception:
-            # If anything goes wrong, revert to the original rcs.
-            dict.update(rcParams, self._orig)
+            self.__fallback()
             raise
 
+    def __fallback(self):
+        # If anything goes wrong, revert to the original rcs.
+        updated_backend = self._orig['backend']
+        dict.update(rcParams, self._orig)
+        # except for the backend.  If the context block triggered resloving
+        # the auto backend resolution keep that value around
+        if self._orig['backend'] is rcsetup._auto_backend_sentinel:
+            rcParams['backend'] = updated_backend
+
     def __enter__(self):
         return self
 
     def __exit__(self, exc_type, exc_value, exc_tb):
-        # No need to revalidate the original values.
-        dict.update(rcParams, self._orig)
+        self.__fallback()
 
 
 def use(arg, warn=True, force=False):
