%matplotlib magic does not respect matplotlib's backend selection mechanism

[anntzer]

(null is an empty profile)

$ MPLBACKEND=tkagg ipython --profile=null
Python 3.6.0 (default, Jan 16 2017, 12:12:55) 
Type "copyright", "credits" or "license" for more information.

IPython 5.2.2 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

IPython profile: null

In [1]: %matplotlib
Using matplotlib backend: Qt5Agg

Ideally %matplotlib would respect MPLBACKEND.

It appears that the problem happens in pylabtools.find_gui_and_backend, namely

        # 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']  ### <- oops

The correct backend can be obtained from matplotlib as matplotlib.get_backend() instead.

[takluyver]

@tacaswell can you comment on what we should be doing here? The suggested solution appears to be equivalent to what the comment just above that line says we're deliberately not doing, but I don't really understand the implications.

[tacaswell]

This should probably be handled on the mpl side where we consume the env variables.

[jasongrout]

@anntzer posted this to gitter:

I tried

 diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
index 84d98a2da..c7981b3a1 100644
--- a/lib/matplotlib/__init__.py
+++ b/lib/matplotlib/__init__.py
@@ -1413,6 +1413,8 @@ try:
     use(os.environ['MPLBACKEND'])
 except KeyError:
     pass
+else:
+    rcParamsOrig['backend'] = os.environ['MPLBACKEND']


 def get_backend():

which would be the obvious solution. IPython appears to select the right backend when calling %matplotlib, but the raised window freezes, which suggests to me that something is wrong with the event loop in that case