[py-dev] issues with stdout redirection, ctypes and py.test

Gordon Wrigley py-dev at tolomea.com
Sat Aug 8 06:43:24 CEST 2009

Since I have no clear concept of what is supposed to happen when a C
function called via ctypes does a printf, I have instead gone through
the C library and arranged to have all the printing routed through a
function pointer which I now setup as a python callback that does a
sys.stdout.write(). Problem solved.

On Fri, Aug 7, 2009 at 8:31 PM, Gordon Wrigley<py-dev at tolomea.com> wrote:
> I need to have a think about what you've said and look at producing a
> minimal reproduction. However I did forget to mention that the prints
> from the python code are fine, it's only the ones from C that are
> affected, in the good case the two sets are interleaved in the py.test
> capture, in the bad case the python prints are in the py.test capture
> and the C prints come out of py.test's stdout.
> Now that I think about it some more I'm inclined to wonder from where
> (and when) the C library acquires it's stdout handle.
> On Fri, Aug 7, 2009 at 8:02 PM, holger krekel<holger at merlinux.eu> wrote:
>> Hi Gordon,
>> On Fri, Aug 07, 2009 at 18:53 +1000, Gordon Wrigley wrote:
>>> My python code talks to a C library via ctypes and it in turn prints
>>> debug to stdout.
>>> And we test this with py.test, which by itself works just fine.
>> by this you mean that it looks roughly like this:
>>    http://paste.pocoo.org/show/132982/
>> right?
>>> But if at the bash prompt we then redirect the output of py.test to a
>>> file suddenly the C debug starts propagating out of py.test.
>> hum, redirecting the above example with "py.test x.py >log" works fine.
>> The following may be interesting to observe (i just did):
>>    def test_hello():
>>        myfile = os.fdopen(1, "w")
>>        myfile.write("hello\n")
>>        myfile.flush()
>>        assert 0
>> This works fine but if i leave out the flush() the write
>> will *not* be captured appropriately.  I guess that it is because
>> of the missing buffering ("os.fdopen(1, 'w', 1)" and no flush works)
>> FYI the pytest_capture plugin resumes capturing for calling
>> into test methods, setup/teardown code and collection
>> respectively and otherwise suspends capturing.  Even if this
>> was modified (i.e. capturing would always happen) i imagine
>> that the missing flush/close could associate the actual write
>> into a different test.
>> Maybe your behaviour is still a different issue or even a bug, though.
>>> We generally run py.test with -v and -x, but removing those didn't
>>> change the behavior at all.
>> that shouldn't change anything related to capturing, indeed.
>>> I'm at a bit of loss as to where to begin with this one, any suggestions?
>> maybe my above considerations help a bit and you can maybe
>> come up with a breaking example or fix things on your side?
>> I want to do a 1.0.1 soon with some capturing fixes (related
>> to unicode handling, though) and i'd like to include any other
>> related improvements or add to the docs.
>> best,
>> holger
>>> gordonw at gohma:~$ uname -a
>>> Linux gohma 2.6.28-14-generic #47-Ubuntu SMP Sat Jul 25 00:28:35 UTC
>>> 2009 i686 GNU/Linux
>>> gordonw at gohma:~$ python
>>> Python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41)
>>> [GCC 4.3.3] on linux2
>>> Type "help", "copyright", "credits" or "license" for more information.
>>> >>> import py
>>> >>> py.version
>>> '1.0.0'
>>> Regards,
>>> Gordon
>>> _______________________________________________
>>> py-dev mailing list
>>> py-dev at codespeak.net
>>> http://codespeak.net/mailman/listinfo/py-dev
>> --
>> Metaprogramming, Python, Testing: http://tetamap.wordpress.com
>> Python, PyPy, pytest contracting: http://merlinux.eu

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