TestCases are supposed to be derived from 
unittest.TestCase; however, if they are derived from 
unittest.TestSuite, the traceback is inexplicable and 
hard to diagnose:


Traceback (most recent call last):
  File "C:/pydev/tmp.py", line 10, in -toplevel-
    unittest.TextTestRunner(verbosity=2).run(suite)
  File "C:\PY24\lib\unittest.py", line 690, in run
    test(result)
  File "C:\PY24\lib\unittest.py", line 423, in __call__
    test(result)
  File "C:\PY24\lib\unittest.py", line 423, in __call__
    test(result)
  File "C:\PY24\lib\unittest.py", line 423, in __call__
    test(result)
TypeError: 'str' object is not callable

Let's either improve the error message or make it 
possible to derive from TestSuite.

The above traceback is produced by the following script:

import unittest

class TestStats(unittest.TestSuite):

    def testtwo(self):
        self.assertEqual(2,2)

suite = unittest.TestSuite()
suite.addTest(unittest.makeSuite(TestStats))
unittest.TextTestRunner(verbosity=2).run(suite)

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patch is at #1061904

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Well, the error message above has been fixed, but here's
another equally confusing variant:

import unittest

class TestStats(unittest.TestSuite):
    def testtwo(self):
        self.assertEqual(2,2)

suite = unittest.TestSuite([TestStats])
unittest.TextTestRunner(verbosity=2).run(suite)

giving:

Traceback (most recent call last):
  File "test-foo.py", line 8, in ?
    unittest.TextTestRunner(verbosity=2).run(suite)
  File "/home/johannes/python/Lib/unittest.py", line 695, in run
    test(result)
  File "/home/johannes/python/Lib/unittest.py", line 426, in
__call__
    test(result)
  File "/home/johannes/python/Lib/unittest.py", line 396, in
__init__
    self.addTests(tests)
  File "/home/johannes/python/Lib/unittest.py", line 416, in
addTests
    for test in tests:
TypeError: iteration over non-sequence

so I'm leaving this open.

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Just to be clear: The issue with jlgijsbers' odd error
message (in his 2004-11-07 followup) is not that a TestSuite
is passed to the TestSuite constructor, but rather that he's
passing in a class, not an instance.

The error would show up a bit earlier if a check were added
to TestSuite.addTest():

if type(test) == type: raise TypeError('addTest argument 1
must be instance, not class')

I will post that line as a patch if anyone emails me to
request it!

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In fact in jlgijsbers' case (TestSuite class passed to
TestSuite, rather than another bug: TestCase class passed to
TestSuite), that check just pushes the problem back a bit:

import unittest
class TestStats(unittest.TestSuite):
    def testtwo(self):
        self.assertEqual(2,2)
suite = unittest.TestSuite([TestStats('testtwo')])
unittest.TextTestRunner(verbosity=2).run(suite)

TypeError: 'str' object is not callable

Another test could be added to TestSuite addTest():

if not callable(test): raise TypeError('addTest argument 1
must be callable')

Again, though it does makes the error clearer, it seems
debatable that it's worth the change...

Added a few sanity checks in rev. 54207, 54208 (2.5).