On 5 August 2018 at 00:46, Stefan Behnel <
stefan_ml@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.