Fix unpickling of CallbackRegistry on Py2.

In general, callbacks may not be pickled; thus, we simply recreate an empty dictionary at unpickling. In order to ensure that `__setstate__` (which just defers to `__init__`) is called, `__getstate__` must return a truthy value (for pickle protocol>=3, i.e. Py3, the *actual* behavior is that `__setstate__` will be called as long as `__getstate__` does not return `None`, but this is undocumented -- see http://bugs.python.org/issue12290).
matplotlib · NelleV · Jan 24, 2017 · Jan 22, 2017 · Jan 22, 2017 · ef3072edefa18d81514547d652d5c6661cfddcbc
commit ef3072edefa18d81514547d652d5c6661cfddcbc
diff --git a/lib/matplotlib/cbook/__init__.py b/lib/matplotlib/cbook/__init__.py
@@ -291,13 +291,18 @@ def __init__(self):
        self._cid = 0
        self._func_cid_map = {}

+    # In general, callbacks may not be pickled; thus, we simply recreate an
+    # empty dictionary at unpickling.  In order to ensure that `__setstate__`
+    # (which just defers to `__init__`) is called, `__getstate__` must
+    # return a truthy value (for pickle protocol>=3, i.e. Py3, the
+    # *actual* behavior is that `__setstate__` will be called as long as
+    # `__getstate__` does not return `None`, but this is undocumented -- see
+    # http://bugs.python.org/issue12290).
+
    def __getstate__(self):
-        # We cannot currently pickle the callables in the registry, so
-        # return an empty dictionary.
-        return {}
+        return True

    def __setstate__(self, state):
-        # re-initialise an empty callback registry
        self.__init__()

    def connect(self, s, func):

diff --git a/lib/matplotlib/tests/test_cbook.py b/lib/matplotlib/tests/test_cbook.py
@@ -1,6 +1,7 @@
 from __future__ import (absolute_import, division, print_function,
                        unicode_literals)
 import itertools
+import pickle
 from weakref import ref
 import warnings

@@ -283,6 +284,10 @@ def test_callback_complete(self):
    def dummy(self):
        pass

+    def test_pickling(self):
+        assert hasattr(pickle.loads(pickle.dumps(cbook.CallbackRegistry())),
+                       "callbacks")
+

 def test_sanitize_sequence():
    d = {'a': 1, 'b': 2, 'c': 3}