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I tried to put assertions in PyObject_GetAttr(), _PyObject_LookupAttr(), _PyObject_GetMethod() and PyObject_SetAttr() to ensure that the type is ready. It breaks many tests, examples:
 test_buffer test_pickle test_picklebuffer test_pickletools
 test_structmembers
For example, Modules/_testbuffer.c does not initialize its ndarray type.
Example of crash:
-----------------
Objects/object.c:892: PyObject_GetAttr: Assertion "tp->tp_flags & (1UL << 12)" failed
Enable tracemalloc to get the memory block allocation traceback
object address : 0x7f4d89119a00
object refcount : 4
object type : 0x8768c0
object type name: type
object repr : <class 'ndarray'>
Fatal Python error: _PyObject_AssertFailed: _PyObject_AssertFailed
Python runtime state: initialized
Current thread 0x00007f4d96eba740 (most recent call first):
 File "/home/vstinner/python/master/Lib/test/test_picklebuffer.py", line 80 in test_ndarray_2d
 (...)
Extension modules: _testcapi, _testbuffer (total: 2)
-----------------
Another example of crash:
-----------------
Objects/object.c:892: PyObject_GetAttr: Assertion "tp->tp_flags & (1UL << 12)" failed
Enable tracemalloc to get the memory block allocation traceback
object address : 0x7f174ec9aa00
object refcount : 3
object type : 0x8768c0
object type name: type
object repr : <class 'ndarray'>
Fatal Python error: _PyObject_AssertFailed: _PyObject_AssertFailed
Python runtime state: initialized
Current thread 0x00007f175ca42740 (most recent call first):
 File "/home/vstinner/python/master/Lib/test/pickletester.py", line 304 in __reduce_ex__
 (...)
Extension modules: _testcapi, _testbuffer, numpy.core._multiarray_umath, numpy.core._multiarray_tests, numpy.linalg.lapack_lite, numpy.linalg._umath_linalg, numpy.fft._pocketfft_internal, numpy.random._common, numpy.random.bit_generator, numpy.random._bounded_integers, numpy.random._mt19937, numpy.random.mtrand, numpy.random._philox, numpy.random._pcg64, numpy.random._sfc64, numpy.random._generator (total: 16)
-----------------
The C API tries to make the type ready automatically. For example, PyObject_Hash() calls PyType_Ready() if needed and then retry tp_hash:
Py_hash_t
PyObject_Hash(PyObject *v)
{
 PyTypeObject *tp = Py_TYPE(v);
 if (tp->tp_hash != NULL)
 return (*tp->tp_hash)(v);
 /* To keep to the general practice that inheriting
 * solely from object in C code should work without
 * an explicit call to PyType_Ready, we implicitly call
 * PyType_Ready here and then check the tp_hash slot again
 */
 if (tp->tp_dict == NULL) {
 if (PyType_Ready(tp) < 0)
 return -1;
 if (tp->tp_hash != NULL)
 return (*tp->tp_hash)(v);
 }
 /* Otherwise, the object can't be hashed */
 return PyObject_HashNotImplemented(v);
}