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Consider the following example unit test using assertAlmostEqual which uses the places argument as a positional argument, call this places.py:
import unittest
class Tests(unittest.TestCase):
 def test_equal_to_five_decimal_places(self):
 """Check assertAlmostEqual using decimal places"""
 self.assertAlmostEqual(0.123456789, 0.123456, 5)
if __name__ == "__main__":
 runner = unittest.TextTestRunner(verbosity = 2)
 unittest.main(testRunner=runner)
This works fine with Python 2.6.6 (Linux),
$ python2.6 places.py
Check assertAlmostEqual using decimal places ... ok
----------------------------------------------------------------------
Ran 1 test in 0.000s
OK
Trying 2to3 on it reports no changes needed:
$ 2to3 places.py
RefactoringTool: Skipping implicit fixer: buffer
RefactoringTool: Skipping implicit fixer: idioms
RefactoringTool: Skipping implicit fixer: set_literal
RefactoringTool: Skipping implicit fixer: ws_comma
RefactoringTool: No changes to places.py
RefactoringTool: Files that need to be modified:
RefactoringTool: places.py
Trying the test as is under Python 3.1.2 (Linux) fails:
$ python3.1 places.py
test_equal_to_five_decimal_places (__main__.Tests)
Check assertAlmostEqual using decimal places ... ERROR
======================================================================
ERROR: test_equal_to_five_decimal_places (__main__.Tests)
Check assertAlmostEqual using decimal places
----------------------------------------------------------------------
Traceback (most recent call last):
 File "places.py", line 5, in test_equal_to_five_decimal_places
 self.assertAlmostEqual(0.123456789, 0.123456, 5)
TypeError: assertAlmostEqual() takes exactly 3 positional arguments (4 given)
----------------------------------------------------------------------
Ran 1 test in 0.001s
FAILED (errors=1)
The test can be fixed to run on Python 3.1 (any Python 2.6) by supplying the places argument by name:
import unittest
class Tests(unittest.TestCase):
 def test_equal_to_five_decimal_places(self):
 """Check assertAlmostEqual using decimal places"""
 self.assertAlmostEqual(0.123456789, 0.123456, places=5)
if __name__ == "__main__":
 runner = unittest.TextTestRunner(verbosity = 2)
 unittest.main(testRunner=runner)
Note http://docs.python.org/library/unittest.html does not explicitly discuss passing places by name vs position. On the other hand, http://docs.python.org/py3k/library/unittest.html explicitly shows the by name form only.
I think the 2to3 script needs to be extended to use the places argument by name.