Merge pull request #4230 from fperez/mpl-backends

Switch correctly to the user's default matplotlib backend after inline.

If '%matplotlib inline' was called first, we'd incorrectly revert to
inline when plain '%matplotlib' was called, instead of loading the
user's default GUI. If the user called '%matplotlib' first (without
'inline') it worked correctly, but not in the other order.

The fix is to read the backend from the original defaults, not from
the runtime data structure.

Requires matplotlib 1.1

docs/source/whatsnew/pr/incompat-mpl-backend.rst

@@ -0,0 +1,4 b''
+We fixed an issue with switching between matplotlib inline and GUI backends,
+but the fix requires matplotlib 1.1 or newer.  So from now on, we consider
+matplotlib 1.1 to be the minimally supported version for IPython. Older
+versions for the most part will work, but we make no guarantees about it.

IPython/core/pylabtools.py

@@ -214,7 +214,11 b' def find_gui_and_backend(gui=None, gui_select=None):'
         # select backend based on requested gui
         backend = backends[gui]
     else:
-        backend = matplotlib.rcParams['backend']
+        # We need to read the backend from the original data structure, *not*
+        # from mpl.rcParams, since a prior invocation of %matplotlib may have
+        # overwritten that.
+        # WARNING: this assumes matplotlib 1.1 or newer!!
+        backend = matplotlib.rcParamsOrig['backend']
         # In this case, we need to find what the appropriate gui selection call
         # should be for IPython, so we can activate inputhook accordingly
         gui = backend2gui.get(backend, None)