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PyObject_Size(), PySequence_Size() and PyMapping_Size() can raise an exception. But not always this is checked after using them. This can lead to a crash. For example:
>>> import io
>>> class R(io.IOBase):
... def readline(self): return None
... 
>>> next(R())
Fatal Python error: a function returned a result with an error set
TypeError: object of type 'NoneType' has no len() 
The above exception was the direct cause of the following exception:
SystemError: <built-in function next> returned a result with an error set
Current thread 0xb749c700 (most recent call first):
 File "<stdin>", line 1 in <module>

Raymond, could you please look at the change in Objects/setobject.c? It makes the code slightly faster (by avoiding few function calls: PyObject_Size(), PySequence_Size()/PyMapping_Size() and sq_length/mp_length), but the main purpose is making clear that no error check is needed after calling PyObject_Size(). I'm not going to include this change in backports.
Xiang suggests larger change that allows to gets rid of one PyDict_CheckExact(). What do you prefer?

New changeset bf623ae8843dc30b28c574bec8d29fc14be59d86 by Serhiy Storchaka in branch 'master':
bpo-30061: Check if PyObject_Size()/PySequence_Size()/PyMapping_Size() (#1096)
https://github.com/python/cpython/commit/bf623ae8843dc30b28c574bec8d29fc14be59d86

New changeset 680fea4067537a9b9c79aadd44a3a19e83cd2dbf by Serhiy Storchaka in branch '3.6':
bpo-30061: Check if PyObject_Size()/PySequence_Size()/PyMapping_Size() (#1096) (#1180)
https://github.com/python/cpython/commit/680fea4067537a9b9c79aadd44a3a19e83cd2dbf

New changeset e63af29c87b44bb7ada5783cd0ff6ee6d4f9c17c by Serhiy Storchaka in branch '3.5':
[3.5] bpo-30061: Check if PyObject_Size()/PySequence_Size()/PyMapping_Size() (GH-1096) (GH-1180) (#1182)
https://github.com/python/cpython/commit/e63af29c87b44bb7ada5783cd0ff6ee6d4f9c17c

New changeset 64aa4df8502ca5d0a8ffb767ff97f625625c758c by Serhiy Storchaka in branch '2.7':
[2.7] bpo-30061: Check if PyObject_Size()/PySequence_Size()/PyMapping_Size() (GH-1096) (GH-1180) (#1183)
https://github.com/python/cpython/commit/64aa4df8502ca5d0a8ffb767ff97f625625c758c