I tried to run numpy test suite on Python 3.6 compiled in debug mode. I got an assertion error. Then I wanted to try the Git version of numpy, but the "Cythonizing sources" step of the numpy installer fails with:
---
Traceback (most recent call last):
  File "bin/cython", line 9, in <module>
    load_entry_point('Cython==0.23.4', 'console_scripts', 'cython')()
  (...)
  File "lib/python3.6/site-packages/Cython/Compiler/Nodes.py", line 6585, in generate_execution_code
    self.body.generate_execution_code(code)
  File "lib/python3.6/site-packages/Cython/Compiler/Nodes.py", line 7026, in generate_execution_code
    fresh_finally_clause().generate_execution_code(code)
  File "lib/python3.6/site-packages/Cython/Compiler/Nodes.py", line 7013, in fresh_finally_clause
    node_copy = copy.deepcopy(node)
  File "/home/haypo/prog/python/default/Lib/copy.py", line 182, in deepcopy
    y = _reconstruct(x, rv, 1, memo)
  (...)
  File "/home/haypo/prog/python/default/Lib/copy.py", line 314, in _reconstruct
    item = deepcopy(item, memo)
  File "/home/haypo/prog/python/default/Lib/copy.py", line 174, in deepcopy
    rv = reductor(4)
TypeError: can't pickle Argument objects
---
Argument class indirectly comes from Cython.Compiler.Nodes.ExprStatNode.

Command to reproduce the bug:
---
tar -xf mtrand.tar.gz  # download mtrand.tar.gz attached to this issue
python -m venv ENV
ENV/bin/python -m pip install cython
ENV/bin/python ENV/bin/cython --fast-fail -o mtrand.c mtrand.pyx
---

It looks like cython behaviour failed after this change:
---
changeset:   99677:b8d108a2a38e
parent:      99675:80d1faa9735d
parent:      99676:0cd2de69fb66
user:        Serhiy Storchaka <storchaka@gmail.com>
date:        Fri Dec 25 21:05:35 2015 +0200
files:       Lib/test/test_csv.py Misc/NEWS Objects/typeobject.c
description:
Issue #22995: Instances of extension types with a state that aren't
subclasses of list or dict and haven't implemented any pickle-related
methods (__reduce__, __reduce_ex__, __getnewargs__, __getnewargs_ex__,
or __getstate__), can no longer be pickled.  Including memoryview.
---

Sorry, I don't understand yet if it's a regression in Python 3.6 or a real bug in Cython.

Oops, my commands to reproduce the bug were wrong.

Commands to reproduce the bug:
---
<start in Python source code directory>

tar -xf mtrand.tar.gz  # download mtrand.tar.gz attached to this issue

./python -m venv ENV
ENV/bin/python -m pip install cython
ENV/bin/python ENV/bin/cython --fast-fail -o mtrand/mtrand.c mtrand/mtrand.pyx
---

May be this is the same issue as reported in msg257646. This was a bug in Cython and it is fixed now.

"May be this is the same issue as reported in msg257646. This was a bug in Cython and it is fixed now."

Oh, I wasn't aware of the issue #22995. I will read it.

The bug isn't fixed yet in the latest release of Cython (0.23.4, released at 2015-10-10). I guess that it's fixed in the development version.

> This was a bug in Cython and it is fixed now.

If the change is deliberate, can you please mention it in "What's New in Python 3.6" doc? Especially in the "Porting to Python 3.6" section:
https://docs.python.org/dev/whatsnew/3.6.html#porting-to-python-3-6

Can you suggest a fix in the doc? I guess that a method should be added: __reduce__(), __reduce_ex__() or __getstate__()?

Cython 0.23.4 was released at 2015-10-10. The fix was applied 2016-01-16. Try to use Cython from trunk. May be this is not exactly the same bug and Cython needs other fix.

The change is deliberate. It helps to find existing bugs.

Except trivial case when your extension class has no any fields in additional to pickleable parent class, the class needs to provide some combinations of __getstate__, __setstate__, __getnewargs__/__getnewargs_ex__ and __reduce__/__reduce_ex__ to be pickleable. This was said in PEP 307. Previously you silently produced invalid pickle and got an exception or invalid object when unpickle it. Now you more likely get an exception at pickling time.

Stefan, could you please check that this issue is fixed and there are no other cases of pickling or copying unpickleable classes?

Our CI build server says it's all fine. The fix will eventually be released, certainly before Py3.6 comes out.

> Our CI build server says it's all fine. The fix will eventually be released, certainly before Py3.6 comes out.

Ok, fine :-)