MNT: handle all descriptors not just properties

tacaswell · tacaswell · commit 335cd7eca3e4 · 2020-06-05T16:24:21.000-04:00
Try very hard to not trigger the descriptors `__get__` methods. As suggested by @eric-wieser
diff --git a/lib/matplotlib/cbook/__init__.py b/lib/matplotlib/cbook/__init__.py
@@ -11,6 +11,7 @@
 import contextlib
 import functools
 import gzip
+import inspect
 import itertools
 import operator
 import os
@@ -2030,6 +2031,10 @@ def _array_patch_perimeters(x, rstride, cstride):
 def _setattr_cm(obj, **kwargs):
     """
     Temporarily set some attributes; restore original state at context exit.
+
+    .. warning ::
+
+        This is not threadsafe.
     """
     sentinel = object()
 
@@ -2038,14 +2043,28 @@ def _setattr_cm(obj, **kwargs):
     # or are shadowing from something later in the resolution order.
     def get_filtered_attr(obj, attr):
         val = getattr(obj, attr, sentinel)
+        # this is the case where we are adding a new attribute, bail now
+        if val is sentinel:
+            return sentinel
 
+        # we are replacing something in the instance dict, we have to restore
+        # it so return the value
         if attr in obj.__dict__:
             return val
-        if isinstance(getattr(type(obj), attr), property):
+
+        # detect when we have Python Descriptors that supports
+        # setting so we will need to restore it
+        #
+        # https://docs.python.org/3/howto/descriptor.html
+        if hasattr(inspect.getattr_static(type(obj), attr), '__set__'):
             return val
 
+        # in all other cases we are shadowing something from deeper in the
+        # resolution order by putting a value in the instance dict so we
+        # will want to remove it at the end.
         return sentinel
 
+    # grab the original values or sentinel so we can clean up
     origs = [(attr, get_filtered_attr(obj, attr)) for attr in kwargs]
     try:
         for attr, val in kwargs.items():
diff --git a/lib/matplotlib/tests/test_cbook.py b/lib/matplotlib/tests/test_cbook.py
@@ -677,27 +677,29 @@ def prop(self, val):
     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.
-    #
-    # https://docs.python.org/3/howto/descriptor.html#functions-and-methods
-    assert a.meth is not a.meth
-    # normal attribute should give you back the same
-    # instance every time
-    assert a.aardvark is a.aardvark
-    # 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'
+    def verify_pre_post_state(obj):
+        # 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.
+        #
+        # https://docs.python.org/3/howto/descriptor.html#functions-and-methods
+        assert obj.meth is not obj.meth
+        # normal attribute should give you back the same
+        # instance every time
+        assert obj.aardvark is obj.aardvark
+        assert a.aardvark == 'aardvark'
+        # and our property happens to give the same instance every time
+        assert obj.prop is obj.prop
+        assert obj.cls_level is A.cls_level
+        assert obj.override == 'override'
+        assert not hasattr(obj, 'extra')
 
+    a = B()
+    verify_pre_post_state(a)
     with cbook._setattr_cm(
             a,
-            aardvark='moose', meth=lambda: None, prop='b', cls_level='bob',
-            override='boo'
+            aardvark='moose', meth=lambda: None, prop='b',
+            cls_level='bob', override='boo', extra='extra'
     ):
         # because we have set a lambda, it is normal attribute access
         # and the same every time
@@ -707,11 +709,5 @@ class B(A):
         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'
+        assert a.extra == 'extra'
+    verify_pre_post_state(a)
