Calling copy.copy on a threading.local subclass copies attributes over correctly in Python 2.7, but creates an empty object in Python 3.3-3.5 and fails with a pickle-related error in 3.6.

Marking this as a release blocker and assigning to Ned because this appears to be a regression in 3.6. However, given that the behavior in previous Python 3 versions isn't very useful either, I'd personally not want to block 3.6 on it.

I haven't yet looked at code to figure out what is causing the differences in behavior. I couldn't find any tracker issues related to copying or pickling threading.local objects, but may have missed something.

$ cat thread_local_copy.py 
import copy
import threading

class Obj(threading.local):
    def __init__(self):
        self.x = 3

o = Obj()
o2 = copy.copy(o)
assert hasattr(o2, 'x')
$ python2.7 thread_local_copy.py 
$ python3.3 thread_local_copy.py 
Traceback (most recent call last):
  File "thread_local_copy.py", line 10, in <module>
    assert hasattr(o2, 'x')
AssertionError
$ python3.4 thread_local_copy.py 
Traceback (most recent call last):
  File "thread_local_copy.py", line 10, in <module>
    assert hasattr(o2, 'x')
AssertionError
$ python3.5 thread_local_copy.py 
Traceback (most recent call last):
  File "thread_local_copy.py", line 10, in <module>
    assert hasattr(o2, 'x')
AssertionError
$ ./python.exe -V
Python 3.6.0+
$ ./python.exe thread_local_copy.py 
Traceback (most recent call last):
  File "thread_local_copy.py", line 9, in <module>
    o2 = copy.copy(o)
  File "/Users/Jelle/code/cpython/Lib/copy.py", line 96, in copy
    rv = reductor(4)
TypeError: can't pickle Obj objects