Simplify CallbackRegistry pickling.

anntzer · anntzer · commit 3d50903ceeb7 · 2020-01-14T14:39:43.000+01:00
The deleted comment was really only relevant to Py2.
Preserving _cid_gen (which is picklable) is necessary to later enable
careful pickling of select callbacks, such as 3d mouse handlers
(otherwise, the same cid may be assigned twice, one before pickling and
once after unpickling).
diff --git a/lib/matplotlib/cbook/__init__.py b/lib/matplotlib/cbook/__init__.py
@@ -151,29 +151,18 @@ def h(exc):
     """
 
     # We maintain two mappings:
-    #   callbacks: signal -> {cid -> callback}
-    #   _func_cid_map: signal -> {callback -> cid}
-    # (actually, callbacks are weakrefs to the actual callbacks).
+    #   callbacks: signal -> {cid -> weakref-to-callback}
+    #   _func_cid_map: signal -> {weakref-to-callback -> cid}
 
     def __init__(self, exception_handler=_exception_printer):
         self.exception_handler = exception_handler
         self.callbacks = {}
         self._cid_gen = itertools.count()
         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):
-        return {'exception_handler': self.exception_handler}
-
-    def __setstate__(self, state):
-        self.__init__(**state)
+        # In general, callbacks may not be pickled, so we just drop them.
+        return {**vars(self), "callbacks": {}, "_func_cid_map": {}}
 
     def connect(self, s, func):
         """Register *func* to be called when signal *s* is generated.
