[Python-Dev] Use of Cython
ronaldoussoren at mac.com
Sun Aug 5 04:06:43 EDT 2018
> On 5 Aug 2018, at 03:15, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 5 August 2018 at 00:46, Stefan Behnel <stefan_ml at behnel.de> wrote:
>> Antoine Pitrou schrieb am 04.08.2018 um 15:57:
>>> Actually, I think testing the C API is precisely the kind of area where
>>> you don't want to involve a third-party, especially not a moving target
>>> (Cython is actively maintained and generated code will vary after each
>>> new Cython release). Besides, Cython itself calls the C API, which
>>> means you might end up involuntarily testing the C API against itself.
>>> If anything, testing the C API using ctypes or cffi would probably be
>>> more reasonable... assuming we get ctypes / cffi to compile everywhere,
>>> which currently isn't the case.
>> I agree that you would rather not want to let Cython (or another tool)
>> generate the specific code that tests a specific C-API call, but you could
>> still use Cython to get around writing the setup, validation and unittest
>> boilerplate code in C. Basically, a test could then look something like
>> this (probably works, although I didn't test it):
>> from cpython.object cimport PyObject
>> from cpython.list cimport PyList_Append
>> def test_PyList_Append_on_empty_list():
>> # setup code
>> l = 
>> assert len(l) == 0
>> value = "abc"
>> pyobj_value = <PyObject*> value
>> refcount_before = pyobj_value.ob_refcnt
>> # conservative test call, translates to the expected C code,
>> # although with exception propagation if it returns -1:
>> errcode = PyList_Append(l, value)
>> # validation
>> assert errcode == 0
>> assert len(l) == 1
>> assert l is value
>> assert pyobj_value.ob_refcnt == refcount_before + 1
>> If you don't want the exception handling, you can define your own
>> declaration of PyList_Append() that does not have it. But personally, I'd
>> rather use try-except in my test code than manually taking care of cleaning
>> up (unexpected) exceptions.
> Exactly, that's the kind of thing I had in mind. At the moment,
> writing a new dedicated C API test requires designing 4 things:
> 1. The test case itself (what action to take, which assertions to make about it)
> 2. The C code to make the API call you want to test
> 3. The Python->C interface for the test case from 1 to pass test
> values in to the code from 2
> 4. The C->Python interface to get state of interest from 2 back to the
> test case from 1
> If we were able to use Cython to handle 3 & 4 rather than having to
> hand craft it for every test, then I believe it would significantly
> lower the barrier to testing the C API directly rather than only
> testing it indirectly through the CPython implementation.
> Having such a test suite available would then hopefully make it easier
> for other implementations to provide robust emulations of the public C
> ctypes & cffi likely wouldn't help as much in the case, since they
> don't eliminate the need to come up with custom code for parts 3 & 4,
> they just let you write that logic in Python rather than C.
I’m not sure if I understand this, ctypes and cffi are used to access C APIs without writing C code including the CPython API (see for example <https://github.com/abarnert/superhackyinternals/blob/master/internals.py <https://github.com/abarnert/superhackyinternals/blob/master/internals.py>>).
The code code below should be mostly equivalent to the Cython example posted earlier:
from ctypes import pythonapi
_fields_ = (
pythonapi.PyList_Append.argtypes = [ctypes.py_object, ctypes.py_object]
# setup code
l = 
assert len(l) == 0
value = "abc"
refcount_before = refcount(value)
errcode = pythonapi.PyList_Append(l, value)
assert errcode == 0
assert len(l) == 1
assert l is value
assert refcount(value) == refcount_before + 1
I write “mostly” because I rarely use ctypes and am not 100% sure that I use the API correctly.
A problem with using ctypes is that this tests the ABI and to the API, which for example means you cannot test C macros this way.
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