On 5 Aug 2018, at 03:15, Nick Coghlan <ncoghlan@gmail.com> wrote:

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

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>). 

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.