I have tried this on several debian unstable releases, and get the following 3 failures

lac at smartwheels:~$ lsb_release -a
 LSB Version:	core-2.0-amd64:core-2.0-noarch:core-3.0-amd64:core-3.0-noarch:core-3.1-amd64:core-3.1-noarch:core-3.2-amd64:core-3.2-noarch:core-4.0-amd64:core-4.0-noarch:core-4.1-amd64:core-4.1-noarch:security-4.0-amd64:security-4.0-noarch:security-4.1-amd64:security-4.1-noarch
 Distributor ID:	Debian
 Description:		Debian GNU/Linux unstable (sid)
 Release:		unstable
 Codename:		sid

Idle shows my tk version as 8.6.4
python3 -m test -ugui -v test_tk gives 3 failures

= CPython 3.4.3+ (default, Jul 28 2015, 13:17:50) [GCC 4.9.3]
==   Linux-3.16.0-4-amd64-x86_64-with-debian-stretch-sid little-endian
==   hash algorithm: siphash24 64bit
==   /tmp/test_python_7974
Testing with flags: sys.flags(debug=0, inspect=0, interactive=0, optimize=0, dont_write_bytecode=0, no_user_site=0, no_site=0, ignore_environment=0, verbose=0, bytes_warning=0, quiet=0, hash_randomization=1, isolated=0)

test_default (tkinter.test.test_tkinter.test_variables.TestBooleanVar) ... FAIL
test_get (tkinter.test.test_tkinter.test_variables.TestBooleanVar) ... FAIL
test_set (tkinter.test.test_tkinter.test_variables.TestBooleanVar) ... FAIL

======================================================================
FAIL: test_default (tkinter.test.test_tkinter.test_variables.TestBooleanVar)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.4/tkinter/test/test_tkinter/test_variables.py", line 163, in test_default
      self.assertIs(v.get(), False)
      AssertionError: 0 is not False

======================================================================
FAIL: test_get (tkinter.test.test_tkinter.test_variables.TestBooleanVar)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.4/tkinter/test/test_tkinter/test_variables.py", line 167, in test_get
      self.assertIs(v.get(), True)
      AssertionError: 1 is not True

======================================================================
FAIL: test_set (tkinter.test.test_tkinter.test_variables.TestBooleanVar)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.4/tkinter/test/test_tkinter/test_variables.py", line 186, in test_set
      self.assertEqual(self.root.globalgetvar("name"), true)
      AssertionError: 42 != 1

----------------------------------------------------------------------
Ran 660 tests in 3.901s

FAILED (failures=3)
1 test failed:
    test_tk

This looks strange. Current default Tcl in Debian unstable is 8.6 [1]. New Python3 builds depend on libtcl8.6 [2]. The full version of the 8.4 branch is 8.4.20 [3], this is the last release in the 8.4 branch. Perhaps your installation was not updated too long time if you see 8.6.4.

[1] https://packages.debian.org/sid/tcl
[2] https://packages.debian.org/sid/python3-tk
[3] https://packages.debian.org/sid/tcl8.4

Even on Debian Squeeze (oldoldstable) Python 2 (2.6) [1] and Python 3 (3.1) [2] depend on Tcl 8.5 [3]. So I think we can ignore all issues with Tcl/Tk 8.4 on Debian. They are related to very old unsupported Debian versions.

[1] https://packages.debian.org/squeeze/python-tk
[2] https://packages.debian.org/squeeze/python3-tk
[3] https://packages.debian.org/squeeze/tcl8.5

You are misunderstanding me, so originally posting this in a report
that mentioned tk and tcl 8.4 was a bad idea. 

I have tk and tcl 8.6 and am getting these errors.  It has nothing
at all to do with 8.4

lac@fido:~$ apt-cache policy tcl tk
tcl:
  Installed: 8.6.0+8
  Candidate: 8.6.0+8
  Version table:
 *** 8.6.0+8 0
        500 http://mirror.openend.se/debian/ stable/main i386 Packages
        500 http://ftp.se.debian.org/debian/ unstable/main i386 Packages
        500 http://ftp.debian.org/debian/ unstable/main i386 Packages
        100 /var/lib/dpkg/status
tk:
  Installed: 8.6.0+8
  Candidate: 8.6.0+8
  Version table:
 *** 8.6.0+8 0
        500 http://mirror.openend.se/debian/ stable/main i386 Packages
        500 http://ftp.se.debian.org/debian/ unstable/main i386 Packages
        500 http://ftp.debian.org/debian/ unstable/main i386 Packages
        100 /var/lib/dpkg/status
lac@fido:~$

Oh, sorry.

The issue still looks strange to me. It looks as a result of mix Python core, library or tests of different versions.

Could you please test what following commands output?

>>> import tkinter
>>> tcl = tkinter.Tcl()
>>> tcl.getboolean(42)
True
>>> tkinter.BooleanVar.set
<function BooleanVar.set at 0xb6ed5614>

Python 3.4.3+ (default, Jul 28 2015, 13:17:50) 
[GCC 4.9.3] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>import tkinter
>>>tcl = tkinter.Tcl()
>>>tcl.getboolean(42)
42
>>>tkinter.BooleanVar.set
<function BooleanVar.set at 0x7f15b780bea0>
>>>print (tkinter)
<module 'tkinter' from '/usr/lib/python3.4/tkinter/__init__.py'>
>>>import _tkinter
>>>print(_tkinter)
<module '_tkinter' from '/usr/lib/python3.4/lib-dynload_tkinter.cpython-34m-x86_64-linux-gnu.so'>


I have the libpython3.4-testsuite installed

lac@fido:~$  apt-cache policy libpython3.4-testsuite
libpython3.4-testsuite:
  Installed: 3.4.3-8
  Candidate: 3.4.3-8
  Version table:
 *** 3.4.3-8 0
        500 http://ftp.se.debian.org/debian/ unstable/main amd64 Packages
        500 http://ftp.debian.org/debian/ unstable/main amd64 Packages
        100 /var/lib/dpkg/status

I am getting these same errors on multiple machines.  As far as I know every one of them gets their packages from the same place, ftp.se.debian.org

Thanks Laura. Looks as binary _tkinter is out of sync with the library and tests. Failing tests were added in issue15133 together with related changes in Python library (added BooleanVar.set and other changes) and _tkinter (changed getboolean()). If Debian version includes 117f45749359, getboolean() always should return boolean. Otherwise failing tests shouldn't exist.

So this is a debian packaging issue we need to tell the debian package maintainers about?

Yes, this looks as packaging issue. Added Matthias Klose, the Debian package maintainer.

Things work with python3.5 so this is an issue with what debian sid calls python3 (i.e. CPython 3.4.3+ (default, Jul 28 2015, 13:17:50) [GCC 4.9.3])

(as expected) Fails when invoked as python3.4 as well.

Laura, could you please report a bug to Debian?

I could.  Have you heard from doko?  If python3 on debian unstable is about to become python3.5 and things work fine on 3.5 I wonder if it is worth the effort.

Matthias, can you confirm that this is Debian packaging issue?