> On 5 Aug 2018, at 03:15, Nick Coghlan <ncogh...@gmail.com> wrote: > > On 5 August 2018 at 00:46, Stefan Behnel <stefan...@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[0] 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 > API. > > 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: import unittest import ctypes from ctypes import pythonapi class PyObject(ctypes.Structure): _fields_ = ( ('ob_refcnt', ctypes.c_ssize_t), ) pythonapi.PyList_Append.argtypes = [ctypes.py_object, ctypes.py_object] def refcount(v): return PyObject.from_address(id(v)).ob_refcnt def test_PyList_Append_on_empty_list(): # 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[0] 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. Ronald
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