MNT: make _setattr_cm more forgiving

tacaswell · tacaswell · commit 2706fa6d84df · 2020-06-16T15:00:28.000-04:00
Our attempts to identify the set of cases we wanted to support did not correctly capture all relevant cases. This tries to simplify the logic: - if the attribute is in the instance dict, stash and restore it via setattr at the end - if the attribute is not on the object, delete it with delattr at the end - if the object has the attribute, but it is not in the instance dict: - if it is a property, stash and restore the old value - in all other cases assume that setattr will put the value in the instance dict and delattr will do what we want on the way out closes #17646
diff --git a/lib/matplotlib/cbook/__init__.py b/lib/matplotlib/cbook/__init__.py
@@ -2035,20 +2035,28 @@ def _setattr_cm(obj, **kwargs):
     origs = {}
     for attr in kwargs:
         orig = getattr(obj, attr, sentinel)
-
         if attr in obj.__dict__ or orig is sentinel:
+            # if we are pulling from the instance dict or the object
+            # does not have this attribute we can trust the above
             origs[attr] = orig
         else:
+            # if the attribute is not in the instance dict it must be
+            # from the class level
             cls_orig = getattr(type(obj), attr)
+            # if we are dealing with a property (but not an general descriptor)
+            # we want to set the original value back.
             if isinstance(cls_orig, property):
                 origs[attr] = orig
-            elif isinstance(cls_orig, types.FunctionType):
-                origs[attr] = sentinel
+            # otherwise this is _something_ we are going to shadow at
+            # the instance dict level from higher up in the MRO.  We
+            # are going to assume we can delattr(obj, attr) to clean
+            # up after ourselves.  It is possible that this code will
+            # fail if used with an non-property custom descriptor which
+            # implements __set__ (and __delete__ does not act like a
+            # stack).  However, this is an internal tool and we do not
+            # currently have any custom descriptors.
             else:
-                raise ValueError(
-                    f"trying to set {attr} which is not a method, "
-                    "property, or instance level attribute"
-                )
+                origs[attr] = sentinel
 
     try:
         for attr, val in kwargs.items():
diff --git a/lib/matplotlib/tests/test_cbook.py b/lib/matplotlib/tests/test_cbook.py
@@ -666,6 +666,14 @@ def __init__(self):
         def meth(self):
             ...
 
+        @classmethod
+        def classy(klass):
+            ...
+
+        @staticmethod
+        def static():
+            ...
+
         @property
         def prop(self):
             return self._p
@@ -696,6 +704,10 @@ def verify_pre_post_state(obj):
         assert not hasattr(obj, 'extra')
         assert obj.prop == 'p'
         assert obj.monkey == other.meth
+        assert obj.cls_level is A.cls_level
+        assert 'cls_level' not in obj.__dict__
+        assert 'classy' not in obj.__dict__
+        assert 'static' not in obj.__dict__
 
     a = B()
 
@@ -705,7 +717,8 @@ def verify_pre_post_state(obj):
             a, prop='squirrel',
             aardvark='moose', meth=lambda: None,
             override='boo', extra='extra',
-            monkey=lambda: None):
+            monkey=lambda: None, cls_level='bob',
+            classy='classy', static='static'):
         # because we have set a lambda, it is normal attribute access
         # and the same every time
         assert a.meth is a.meth
@@ -715,9 +728,8 @@ def verify_pre_post_state(obj):
         assert a.extra == 'extra'
         assert a.prop == 'squirrel'
         assert a.monkey != other.meth
+        assert a.cls_level == 'bob'
+        assert a.classy == 'classy'
+        assert a.static == 'static'
 
     verify_pre_post_state(a)
-
-    with pytest.raises(ValueError):
-        with cbook._setattr_cm(a, cls_level='bob'):
-            pass
