Hi - I've been experiencing many errors trying to build any version
of Python that will pass its test suite - see issues : #11946 , #11954 -
and now I've been advised to raise bugs about each test failure -
hence this bug. For details of my config and build procedure, please
see : issue #11954 .

So, running the new ./python fails test_argparse :

$ LD_LIBRARY_PATH=`pwd` LD_PRELINK=`pwd`/libpython3.3.so.1.0 \
  ./python /usr/src/cpython/Lib/test/test_argparse.py
...
======================================================================
FAIL: test_failures_many_groups_listargs (__main__.TestFileTypeW)     
----------------------------------------------------------------------
Traceback (most recent call last):                                    
  File "/usr/src/cpython/Lib/test/test_argparse.py", line 216, in wrapper
    test_func(self)                                                      
  File "/usr/src/cpython/Lib/test/test_argparse.py", line 235, in test_failures
    raises(ArgumentParserError, parser.parse_args, args)                       
AssertionError: ArgumentParserError not raised by parse_args                   

======================================================================
FAIL: test_failures_many_groups_sysargs (__main__.TestFileTypeW)      
----------------------------------------------------------------------
Traceback (most recent call last):                                    
  File "/usr/src/cpython/Lib/test/test_argparse.py", line 216, in wrapper
    test_func(self)                                                      
  File "/usr/src/cpython/Lib/test/test_argparse.py", line 235, in test_failures
    raises(ArgumentParserError, parser.parse_args, args)                       
AssertionError: ArgumentParserError not raised by parse_args                   

======================================================================
FAIL: test_failures_no_groups_listargs (__main__.TestFileTypeW)       
----------------------------------------------------------------------
Traceback (most recent call last):                                    
  File "/usr/src/cpython/Lib/test/test_argparse.py", line 216, in wrapper
    test_func(self)                                                      
  File "/usr/src/cpython/Lib/test/test_argparse.py", line 235, in test_failures
    raises(ArgumentParserError, parser.parse_args, args)                       
AssertionError: ArgumentParserError not raised by parse_args                   

======================================================================
FAIL: test_failures_no_groups_sysargs (__main__.TestFileTypeW)        
----------------------------------------------------------------------
Traceback (most recent call last):                                    
  File "/usr/src/cpython/Lib/test/test_argparse.py", line 216, in wrapper
    test_func(self)                                                      
  File "/usr/src/cpython/Lib/test/test_argparse.py", line 235, in test_failures
    raises(ArgumentParserError, parser.parse_args, args)                       
AssertionError: ArgumentParserError not raised by parse_args                   

======================================================================
FAIL: test_failures_one_group_listargs (__main__.TestFileTypeW)       
----------------------------------------------------------------------
Traceback (most recent call last):                                    
  File "/usr/src/cpython/Lib/test/test_argparse.py", line 216, in wrapper
    test_func(self)                                                      
  File "/usr/src/cpython/Lib/test/test_argparse.py", line 235, in test_failures
    raises(ArgumentParserError, parser.parse_args, args)                       
AssertionError: ArgumentParserError not raised by parse_args                   

======================================================================
FAIL: test_failures_one_group_sysargs (__main__.TestFileTypeW)        
----------------------------------------------------------------------
Traceback (most recent call last):                                    
  File "/usr/src/cpython/Lib/test/test_argparse.py", line 216, in wrapper
    test_func(self)                                                      
  File "/usr/src/cpython/Lib/test/test_argparse.py", line 235, in test_failures
    raises(ArgumentParserError, parser.parse_args, args)                       
AssertionError: ArgumentParserError not raised by parse_args                   

----------------------------------------------------------------------
Ran 1608 tests in 5.293s                                              

FAILED (failures=6)
Traceback (most recent call last):
  File "/usr/src/cpython/Lib/test/test_argparse.py", line 4712, in <module>
    test_main()                                                            
  File "/usr/src/cpython/Lib/test/test_argparse.py", line 4704, in test_main
    support.run_unittest(__name__)                                          
  File "/usr/src/cpython/Lib/test/support.py", line 1208, in run_unittest   
    _run_suite(suite)                                                       
  File "/usr/src/cpython/Lib/test/support.py", line 1191, in _run_suite     
    raise TestFailed(err)                                                   
test.support.TestFailed: multiple errors occurred 


Now, as someone with not too recent python programming experience,
I find it very difficult to understand exactly what is being
tested by :

class TestFileTypeW(TempDirMixin, ParserTestCase):
    """Test the FileType option/argument type for writing files"""

    def setUp(self):
        super(TestFileTypeW, self).setUp()
        self.create_readonly_file('readonly')

    argument_signatures = [
        Sig('-x', type=argparse.FileType('w')),
        Sig('spam', type=argparse.FileType('w')),
    ]
    failures = ['-x', '', 'readonly']
    successes = [
        ('foo', NS(x=None, spam=WFile('foo'))),
        ('-x foo bar', NS(x=WFile('foo'), spam=WFile('bar'))),
        ('bar -x foo', NS(x=WFile('foo'), spam=WFile('bar'))),
        ('-x - -', NS(x=sys.stdout, spam=sys.stdout)),
    ]


But it seems at least one bug is self-evident : test_argparse.py's
error messages are essentially meaningless, and contain no useful
information (beyond the name) about what failed or why - they are
all essentially duplicates of :

Traceback (most recent call last):
  File "/usr/src/cpython/Lib/test/test_argparse.py", line 216, in wrapper
    test_func(self)
  File "/usr/src/cpython/Lib/test/test_argparse.py", line 235, in test_failures
    raises(ArgumentParserError, parser.parse_args, args)

So why emit 6 copies of the same meaningless message ? 

I'm curious: how do you expect those error messages to help people
track down the source of the bug when every error message contains
the same data and line numbers, and they are line numbers not
of specific tests, but of some "error handler" routine ?

Aha ! the test succeeds as a non root (super-) user .
This is because as a root user I can override w bit
settings on directories I own: see issue #11956 
for fix I applied to test_import.py to fix same
issue. Thanks !

The six error messages tell you that six different tests failed.  Yes, the failures are probably all due to the same cause, but that's just how unit testing works.  (And yes, the argparse tests are a bit more terse and difficult to understand than many of our tests, but that is because they are trying to economically test a lot of different edge cases.)

The helpers in test_argparse could be enhanced to print the tested string and expected result in case of failure.  Or the real line number.

The attached patch adds the list of args to the output of assertRaises in case of error, e.g.:
FAIL: test_failures_one_group_sysargs (test.test_argparse.TestPositionalsNargsZeroOrMoreNone)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/wolf/dev/py/3.3/Lib/test/test_argparse.py", line 216, in wrapper
    test_func(self)
  File "/home/wolf/dev/py/3.3/Lib/test/test_argparse.py", line 236, in test_failures
    parser.parse_args(args)
AssertionError: ArgumentParserError not raised : ['foo', 'bar']

The test names are:

FAIL: test_failures_many_groups_listargs (__main__.TestFileTypeW)     
FAIL: test_failures_many_groups_sysargs (__main__.TestFileTypeW)      
FAIL: test_failures_no_groups_listargs (__main__.TestFileTypeW)       
FAIL: test_failures_no_groups_sysargs (__main__.TestFileTypeW)        
FAIL: test_failures_one_group_listargs (__main__.TestFileTypeW)       
FAIL: test_failures_one_group_sysargs (__main__.TestFileTypeW)    

So they differ by [many_groups, no_groups, one_group] and [listargs, sysargs]

There are about 170 tests in test_argparse that get multiplied by 6 in this way.  If this replication was removed it would cut the number of tests and time to nearly a third.

This replication is not useful.  listargs and sysargs differ only in how a list of arguments is passed to parse_args, either directly, or via sys.argv.  It should be sufficient to test these alternatives once, not 170 times.

'one group' creates an argument group, and adds all arguments to that.  'many groups' creates an argument group for each added argument.  But argument groups are not used for 'parse_args'.  They are only used when formatting help.  By default all the arguments of parser are already put into one of two groups, 'optional arguments' or 'positional arguments'.

There are tests for the help formatting, but they don't use this ParserTesterMetaclass.  

I would recommend removing this test replication.  It is both unnecessary and a source of confusion.  The simplest would be to change the end of the ParserTesterMetaclass.  

    class ParserTesterMetaclass
        def __init__()
        ....
            for add_arguments in [no_groups]:
                for parse_args in [listargs]:
                    AddTests(cls, add_arguments, parse_args)

I suspect there is some confusion among users and developers as to what argument groups do.  

As for the TestFileTypeW error, I suspect it has to do with file writing privileges.  I haven't encountered it, but I running tests in my own directory.

@Ezio will you be following this up, specifically msg193970 ?

New changeset 9a410ae785ff by Ezio Melotti in branch '3.4':
#11955: show the list of args in case of error in test_argparse.
http://hg.python.org/cpython/rev/9a410ae785ff

New changeset 25e634756f79 by Ezio Melotti in branch 'default':
#11955: merge with 3.4.
http://hg.python.org/cpython/rev/25e634756f79

I applied my patch, but that doesn't fix the original issue -- it just provides a better error message in case of failure.

I can not reproduce failures. Are they still reproducible?

As we've not seen any issues with this on the buildbots, I'm going ahead and closing it.