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Currently, PyType_Ready() is called late on many static types, which can lead to bugs. For example, PyObject_SetAttr() can fail if the type of the object is not ready yet. PyType_Read() is responsible to initialize tp_getattro and tp_setattro members of the PyTypeObject structure.
We must explicitly initialize all static types before using them.
Moreover, many static types initialize explicitly :
 tp_getattro = PyObject_GenericGetAttr
and:
 tp_setattro = PyObject_GenericSetAttr
whereas it's the default implementation. They can be omitted.
I created this issue as a placeholder for multiple changes to modify how types implemented in C are initialized.

New changeset a518099078c8ae51860009fb801db897af9eed62 by Victor Stinner in branch 'master':
bpo-43770: Sort types in _PyTypes_Init() (GH-25263)
https://github.com/python/cpython/commit/a518099078c8ae51860009fb801db897af9eed62

New changeset df5dc1c7a536abc2b497cb9506f8a37949838309 by Victor Stinner in branch 'master':
bpo-43770: _PyTypes_Init() inits more static types (GH-25265)
https://github.com/python/cpython/commit/df5dc1c7a536abc2b497cb9506f8a37949838309

New changeset b98eba5bc2ffbe7a0ed49d540ebc4f756ae61985 by Victor Stinner in branch 'master':
bpo-43770: _PyTypes_Init() inits _PyAnextAwaitable_Type (GH-25266)
https://github.com/python/cpython/commit/b98eba5bc2ffbe7a0ed49d540ebc4f756ae61985

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);
}

PR 25275: this is a subtle difference if PyTypeObject.tp_setattro is set statically to PyObject_GenericSetAttr() or if it's inherited by PyType_Ready().
Reference (master)::
* BaseException.__dict__['__setattr__'] = <slot wrapper '__setattr__' of 'BaseException' objects>
* BaseException.__setattr__ = <slot wrapper '__setattr__' of 'BaseException' objects>
With the PR:
* no '__setattr___' in BaseException.__dict__
* BaseException.__setattr__ = <slot wrapper '__setattr__' of 'object' objects>
Because of that, doctest.DocTestFinder().find(builtins) returns less items, and so test_doctest fails.

New changeset ecf14e6557c6e4f7af9c0d6460d31fe121c22553 by Victor Stinner in branch 'master':
bpo-43770: Refactor type_new() function (GH-25325)
https://github.com/python/cpython/commit/ecf14e6557c6e4f7af9c0d6460d31fe121c22553

New changeset 53114ffef1d4facf9aa5545e711abbbda66f672a by Victor Stinner in branch 'master':
bpo-43770: Refactor PyType_Ready() function (GH-25336)
https://github.com/python/cpython/commit/53114ffef1d4facf9aa5545e711abbbda66f672a

New changeset 65f058eb081c9e1fe44115d1ac7966067e3650c7 by Victor Stinner in branch 'master':
bpo-43770: Reorder type_ready() (GH-25373)
https://github.com/python/cpython/commit/65f058eb081c9e1fe44115d1ac7966067e3650c7

New changeset a328d73843cfd42d2aee1434c78df1ef2845931a by Victor Stinner in branch 'master':
bpo-43770: Cleanup type_ready() (GH-25388)
https://github.com/python/cpython/commit/a328d73843cfd42d2aee1434c78df1ef2845931a

If I modify type_ready() to call type_ready_inherit() before type_ready_fill_dict(), some types create a between between their C slot (tp_init) and the Python API in tp_dict (tp_dict["__init__"]).
Example with importlib:
class FileLoader(...):
 def __init__(...):
 ...
=> FileLoader.tp_init = slot_tp_init
class SourceFileLoader(FileLoader):
 ...
When PyType_Ready() is called on SourceFileLoader, we get:
* SourceFileLoader.tp_base = FileLoader
* SourceFileLoader.tp_init = NULL
* SourceFileLoader.tp_dict has no "__init__" key
When inherit_slots() is called, SourceFileLoader.tp_init is set to slot_tp_init().
When add_operators() is called, SourceFileLoader.tp_dict["__init__"] is set to PyDescr_NewWrapper(slot_tp_init).
Problem: we a loop! tp_dict["__init__"] => slot_tp_init => tp_dict["__init__"]

New changeset 2a396d65b8e63300ff05e217adacf0765c502ba3 by Victor Stinner in branch 'main':
bpo-43770: Cleanup PyModuleDef_Init() (GH-26879)
https://github.com/python/cpython/commit/2a396d65b8e63300ff05e217adacf0765c502ba3

New changeset dd3adc013b21ec1338bb5fea2e2c04a4fc650306 by Victor Stinner in branch 'main':
bpo-43770: Cleanup _PyObject_GetMethod() (GH-26946)
https://github.com/python/cpython/commit/dd3adc013b21ec1338bb5fea2e2c04a4fc650306

The code is way more complicated than what I expected. I hope that my work on this issue will help future developers who will try to understand the code. But I prefer to stop the refactoring at this point. I pushed enough changes ;-)