The attached example unit test file shows that assertDictContainsSubset cannot handle error messages that need to show integer keys. Below is the output of the test suite, where "test_mixed_keys_fail" has an error (code mistake), while "test_text_keys_fail" produces a failure (result mistake) as expected.

C:\notes>C:\Python31\python.exe test_one_and_one.py
.E.F
======================================================================
ERROR: test_mixed_keys_fail (__main__.Test_one_and_one)
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Traceback (most recent call last):
  File "test_one_and_one.py", line 30, in test_mixed_keys_fail
    self.assertDictContainsSubset({3: "this does not exist"}, self.dict_with_mix
ed_keys)
  File "C:\python31\lib\unittest.py", line 908, in assertDictContainsSubset
    standardMsg = 'Missing: %r' % ','.join(missing)
TypeError: sequence item 0: expected str instance, int found

======================================================================
FAIL: test_text_keys_fail (__main__.Test_one_and_one)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test_one_and_one.py", line 33, in test_text_keys_fail
    self.assertDictContainsSubset({"3": "this does not exist"}, self.dict_with_t
ext_keys)
AssertionError: Missing: '3'

----------------------------------------------------------------------
Ran 4 tests in 0.010s

FAILED (failures=1, errors=1)