ENH: restore class level attributes by un-shadowing in _setattr_cm

tacaswell · tacaswell · commit 9abe1408315b · 2020-06-04T21:10:06.000-04:00
diff --git a/lib/matplotlib/cbook/__init__.py b/lib/matplotlib/cbook/__init__.py
@@ -24,7 +24,6 @@
 import types
 import warnings
 import weakref
-import inspect
 
 import numpy as np
 
@@ -2034,13 +2033,20 @@ def _setattr_cm(obj, **kwargs):
     """
     sentinel = object()
     origs = [(attr, getattr(obj, attr, sentinel)) for attr in kwargs]
-    # When you access a Python method the function is bound
-    # to the object at access time so you get a new instance
-    # of MethodType every time so replace them with sentinel
-    #
-    # https://docs.python.org/3/howto/descriptor.html#functions-and-methods
-    origs = [(attr, _ if not inspect.ismethod(_) else sentinel)
-             for attr, _ in origs]
+
+    # we only want to set back things that are in the instance dict or
+    # are properties.  Everything else we are either introducing to begin with
+    # or are shadowing from something later in the resolution order.
+    def filter_restores(obj, attr, val):
+        if attr in obj.__dict__:
+            return True
+
+        if isinstance(getattr(type(obj), attr), property):
+            return True
+        return False
+
+    origs = [(attr, val if filter_restores(obj, attr, val) else sentinel)
+             for attr, val in origs]
     try:
         for attr, val in kwargs.items():
             setattr(obj, attr, val)
diff --git a/lib/matplotlib/tests/test_cbook.py b/lib/matplotlib/tests/test_cbook.py
@@ -655,8 +655,12 @@ def test_check_shape(target, test_shape):
 
 def test_setattr_cm():
     class A:
+
+        cls_level = object()
+        override = object()
         def __init__(self):
             self.aardvark = 'aardvark'
+            self.override = 'override'
             self._p = 'p'
 
         def meth(self):
@@ -670,7 +674,10 @@ def prop(self):
         def prop(self, val):
             self._p = val
 
-    a = A()
+    class B(A):
+        ...
+
+    a = B()
     # When you access a Python method the function is bound
     # to the object at access time so you get a new instance
     # of MethodType every time.
@@ -683,19 +690,28 @@ def prop(self, val):
     # and our property happens to give the same instance every time
     assert a.prop is a.prop
 
+    assert a.cls_level is A.cls_level
+
+    assert a.override == 'override'
+
     with cbook._setattr_cm(
             a,
-            aardvark='moose', meth=lambda: None, prop='b'
+            aardvark='moose', meth=lambda: None, prop='b', cls_level='bob',
+            override='boo'
     ):
         # because we have set a lambda, it is normal attribute access
         # and the same every time
         assert a.meth is a.meth
         assert a.aardvark is a.aardvark
         assert a.aardvark == 'moose'
         assert a.prop == 'b'
+        assert a.cls_level == 'bob'
+        assert a.override == 'boo'
 
     # check that we get different MethodType instances each time
     assert a.meth is not a.meth
     assert a.aardvark is a.aardvark
     assert a.aardvark == 'aardvark'
     assert a.prop is a.prop
+    assert a.cls_level is A.cls_level
+    assert a.override == 'override'
