matplotlib · dstansby · Apr 12, 2017 · Mar 29, 2017
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -118,17 +118,18 @@ def add(self, key, a):
         # All the error checking may be unnecessary; but this method
         # is called so seldom that the overhead is negligible.
         if not isinstance(a, Axes):
-            raise ValueError("second argument, %s, is not an Axes" % a)
+            raise ValueError("second argument, {!r}, is not an Axes".format(a))
         try:
             hash(key)
         except TypeError:
-            raise ValueError("first argument, %s, is not a valid key" % key)
+            raise ValueError(
+                "first argument, {!r}, is not a valid key".format(key))
 
         a_existing = self.get(key)
         if a_existing is not None:
             Stack.remove(self, (key, a_existing))
             warnings.warn(
-                "key %s already existed; Axes is being replaced" % key)
+                "key {!r} already existed; Axes is being replaced".format(key))
             # I don't think the above should ever happen.
 
         if a in self:

diff --git a/lib/matplotlib/transforms.py b/lib/matplotlib/transforms.py
@@ -1156,11 +1156,9 @@ def __radd__(self, other):
         raise TypeError(
             "Can not add Transform to object of type '%s'" % type(other))
 
-    def __eq__(self, other):
-        # equality is based on transform object id. Hence:
-        # Transform() != Transform().
-        # Some classes, such as TransformWrapper & AffineBase, will override.
-        return self is other
+    # Equality is based on object identity for `Transform`s (so we don't
+    # override `__eq__`), but some subclasses, such as TransformWrapper &
+    # AffineBase, override this behavior.
 
     def _iter_break_from_left_to_right(self):
         """