Thanks by install do you mean running the test with an installed version of python 3.7.0 from package manager or as a downloaded binary ? I can reproduce it with an installed version from anaconda but the binary compiled on the latest 3.7 branch works fine. Unfortunately, I can't compile tcl 8.6.8 after several tries. I think the test was added after the release of 3.7.0 is made thus causing failure on the released binary but works fine with the compiled one. If the above is correct then the test will also fail with latest 3.6.6 release.

# Test added : June 30, 2018 (https://github.com/python/cpython/commit/42ea5226642c2a416b32278914fa4738093298ff)
# 3.7.0 final : June 27, 2018 (https://github.com/python/cpython/commit/1bf9cc509326bc42cd8cb1650eb9bf64550d817e)

# Running test with installed version of python 3.7.0

➜  cpython git:(25326de) python3.7
Python 3.7.0 (default, Jun 28 2018, 02:32:19)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

➜  cpython git:(25326de) python3.7 Lib/test/test_tcl.py
patchlevel = 8.6.5
testCall (__main__.TclTest) ... ok
testCallException (__main__.TclTest) ... ok
testCallException2 (__main__.TclTest) ... ok
testEval (__main__.TclTest) ... ok
testEvalException (__main__.TclTest) ... ok
testEvalException2 (__main__.TclTest) ... ok
testEvalFile (__main__.TclTest) ... ok
testEvalFileException (__main__.TclTest) ... ok
testGetVar (__main__.TclTest) ... ok
testGetVarArray (__main__.TclTest) ... ok
testGetVarArrayException (__main__.TclTest) ... ok
testGetVarException (__main__.TclTest) ... ok
testLoadWithUNC (__main__.TclTest) ... skipped 'Requires Windows'
testPackageRequireException (__main__.TclTest) ... ok
testSetVar (__main__.TclTest) ... ok
testSetVarArray (__main__.TclTest) ... ok
testUnsetVar (__main__.TclTest) ... ok
testUnsetVarArray (__main__.TclTest) ... ok
testUnsetVarException (__main__.TclTest) ... ok
test_booleans (__main__.TclTest) ... ok
test_eval_null_in_result (__main__.TclTest) ... ok
test_evalfile_null_in_result (__main__.TclTest) ... ok
test_expr_bignum (__main__.TclTest) ... ok
test_exprboolean (__main__.TclTest) ... ok
test_exprdouble (__main__.TclTest) ... ok
test_exprlong (__main__.TclTest) ... ok
test_exprstring (__main__.TclTest) ... ok
test_getboolean (__main__.TclTest) ... ok
test_getdouble (__main__.TclTest) ... ok
test_getint (__main__.TclTest) ... ok
test_join (__main__.TclTest) ... FAIL
test_new_tcl_obj (__main__.TclTest) ... ok
test_passing_values (__main__.TclTest) ... ok
test_split (__main__.TclTest) ... ok
test_splitdict (__main__.TclTest) ... ok
test_splitlist (__main__.TclTest) ... ok
test_user_command (__main__.TclTest) ... ok
testFlattenLen (__main__.TkinterTest) ... ok
test_huge_string_builtins (__main__.BigmemTclTest) ... skipped 'not enough memory: 4.0G minimum needed'
test_huge_string_builtins2 (__main__.BigmemTclTest) ... skipped 'not enough memory: 12.0G minimum needed'
test_huge_string_call (__main__.BigmemTclTest) ... skipped 'not enough memory: 10.0G minimum needed'

======================================================================
FAIL: test_join (__main__.TclTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "Lib/test/test_tcl.py", line 670, in test_join
    check('{spam}')
  File "Lib/test/test_tcl.py", line 660, in check
    self.assertEqual(unpack(unpack(join([[value]]))), value)
AssertionError: 'spam' != '{spam}'
- spam
+ {spam}
? +    +


----------------------------------------------------------------------
Ran 41 tests in 0.108s

FAILED (failures=1, skipped=4)
Traceback (most recent call last):
  File "Lib/test/test_tcl.py", line 752, in <module>
    test_main()
  File "Lib/test/test_tcl.py", line 749, in test_main
    support.run_unittest(TclTest, TkinterTest, BigmemTclTest)
  File "/usr/lib/python3.7/test/support/__init__.py", line 1972, in run_unittest
    _run_suite(suite)
  File "/usr/lib/python3.7/test/support/__init__.py", line 1891, in _run_suite
    raise TestFailed(err)
test.support.TestFailed: Traceback (most recent call last):
  File "Lib/test/test_tcl.py", line 670, in test_join
    check('{spam}')
  File "Lib/test/test_tcl.py", line 660, in check
    self.assertEqual(unpack(unpack(join([[value]]))), value)
AssertionError: 'spam' != '{spam}'
- spam
+ {spam}
? +    +


Thanks