[Cython] cdef class tp_new should call PyBaseObject_Type.tp_new?

[Simon Jagoe]

Hi all,

I recently posted to the cython-users list about this problem. This
email is to submita potential patch to fix the issue. The issue is as
follows:


At Enthought we are busy porting Traits
(http://github.com/enthought/traits) from C to Cython.

An issue has come up that that prevents classes inheriting from a
cdef-class from playing nicely with ABCs.  It is type's tp_new that
checks for abstractmethods that have not been implemented, so if type's
tp_new is bypassed, the check is never run and you can instantiate an
abstract base class.

It is equivalent to this simple example below (taken from
http://stackoverflow.com/questions/20432335). If you replace the simple
class A with a Cython cdef class, the effect is the same.


import abc

class A(object):
    def __new__(cls):
        # self = object.__new__(cls)
        return 42

class B(A):
    __metaclass__ = abc.ABCMeta

    @abc.abstractmethod
    def foo(self):
        pass

b = B()  # No exception.


Thanks,
Simon
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