The following code causes an assertion failure:
import ctypes
class BadStruct(ctypes.Structure):
    _fields_ = []
    _anonymous_ = ['foo']
    foo = None


this is because MakeAnonFields() (in Modules/_ctypes/stgdict.c) goes over the
names specified in _anonymous_, and looks each name up in the class by calling
PyObject_GetAttr().
in case an attribute of such a name is found (i.e. PyObject_GetAttr() succeeded),
MakeAnonFields() assumes that the attribute was created by MakeFields(), so it
asserts the type of the attribute is PyCField_Type.

however, PyObject_GetAttr() would succeed also in case it is a normal attribute
specified by the user, but isn't specified in _fields_, as in the code above.
in such a case, the type of the attribute is not PyCField_Type, and so the
assertion fails.

New changeset 30b61b51e05d2d43e8e2e783b0a9df738535423b by Serhiy Storchaka (Oren Milman) in branch 'master':
bpo-31490: Fix an assertion failure in ctypes in case an _anonymous_ attr is defined only outside _fields_. (#3615)
https://github.com/python/cpython/commit/30b61b51e05d2d43e8e2e783b0a9df738535423b

New changeset bdb215b18a42360b6a9c82876fa71f19ca1a416d by Serhiy Storchaka (Miss Islington (bot)) in branch '3.6':
[3.6] bpo-31490: Fix an assertion failure in ctypes in case an _anonymous_ attr is defined only outside _fields_. (GH-3615) (#3774)
https://github.com/python/cpython/commit/bdb215b18a42360b6a9c82876fa71f19ca1a416d

New changeset 9bfa55bfea60553831e99dbbca6a7c7d1065fd4b by Serhiy Storchaka (Miss Islington (bot)) in branch '2.7':
[2.7] bpo-31490: Fix an assertion failure in ctypes in case an _anonymous_ attr is defined only outside _fields_. (GH-3615) (#3780)
https://github.com/python/cpython/commit/9bfa55bfea60553831e99dbbca6a7c7d1065fd4b

In 2.7 "%U" is not recognized in the format string.

>>> class Name(Structure):
...     _fields_ = []
...     _anonymous_ = ["x"]
...     x = 42
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: '%U' is specified in _anonymous_ but not in _fields_

New changeset fb3bb8d5d5d70acaaa0fdec15c137544fdd4463f by Serhiy Storchaka (Oren Milman) in branch '2.7':
[2.7] bpo-31490: Fix an assertion failure in ctypes in case an _anonymous_ attr is defined only outside _fields_. (GH-3615) (#3952)
https://github.com/python/cpython/commit/fb3bb8d5d5d70acaaa0fdec15c137544fdd4463f