except dynamic_tuple: fails to catch on Python 3

[jamadden]

Given this code in foo.py:

from __future__ import print_function
to_catch = (AssertionError,)
try:
	raise AssertionError("AssertionError")
except to_catch as e:
	print("Caught", e)
except:
	print("Uncaught!")

Both Python 2 and Python 3 will print 'Caught':

$ python2.7 foo.py
Caught AssertionError
$ python3.6 foo.py
Caught AssertionError

But if we compile foo.py with Cython 0.28.3 and then invoke it, only Python 2 correctly catches the exception:

$ cat setup.py
from setuptools import setup
from Cython.Build import cythonize
from setuptools import Extension

setup(
	name='foo',
	ext_modules=cythonize('foo.py')
)
$ cat bar.py
import foo
$ python2.7 setup.py build_ext -i
...
$ python3.6 setup.py build_ext -i
...
$ mv foo.py foo.py.off # make sure we get the extension
$ python2.7 bar.py
Caught AssertionError
$ python3.6 bar.py
Uncaught!

The catch line compiles into a call to __Pyx_PyErr_GivenExceptionMatches

    /* "foo.py":5
 * try:
 * 	raise AssertionError("AssertionError")
 * except to_catch as e:             # <<<<<<<<<<<<<<
 * 	print("Caught", e)
 * except:
 */
    __Pyx_ErrFetch(&__pyx_t_4, &__pyx_t_5, &__pyx_t_6);
    __pyx_t_7 = __Pyx_GetModuleGlobalName(__pyx_n_s_to_catch); if (unlikely(!__pyx_t_7)) __PYX_ERR(0, 5, __pyx_L4_except_error)
    __Pyx_GOTREF(__pyx_t_7);
    __pyx_t_8 = __Pyx_PyErr_GivenExceptionMatches(__pyx_t_4, __pyx_t_7);

static CYTHON_INLINE int __Pyx_PyErr_GivenExceptionMatches(PyObject *err, PyObject* exc_type) {
    if (likely(err == exc_type)) return 1;
    if (likely(PyExceptionClass_Check(err))) {
        return __Pyx_inner_PyErr_GivenExceptionMatches2(err, NULL, exc_type);
    }
    return PyErr_GivenExceptionMatches(err, exc_type);
}

The definitions of __Pyx_inner_PyErr_GivenExceptionMatches2 are different on Python 2 and 3. On Python 2, it goes directly to PyObject_IsSubclass(err, exc_type), which correctly handles tuples.

On Python 3, though, it calls __Pyx_IsSubtype((PyTypeObject*)err, (PyTypeObject*)exc_type2);

static CYTHON_INLINE int __Pyx_inner_PyErr_GivenExceptionMatches2(PyObject *err, PyObject* exc_type1, PyObject *exc_type2) {
    int res = exc_type1 ? __Pyx_IsSubtype((PyTypeObject*)err, (PyTypeObject*)exc_type1) : 0;
    if (!res) {
        res = __Pyx_IsSubtype((PyTypeObject*)err, (PyTypeObject*)exc_type2);
    }
    return res;
}

That's an incorrect cast in the second parameter, and Pyx_IsSubtype doesn't correctly handle tuples for its second argument (luckily it doesn't crash!). If I modify this function to use PyObject_IsSubclass it works as expected.

[mbuesch]

#2426 might be the same problem as this one.
I bisected the original breakage to 6cc0a3a.
For me, with current master it only fails to catch the exception, if the tuple is not a local.
So something between 6cc0a3a and master slightly improved the situation.

[scoder]

Thanks for the excellent analysis.

[jamadden]

Thanks for the quick fix!