[C API] PyType_GetSlot cannot get tp_name

[fancitron]

BPO	42035
Nosy	@vstinner, @encukou, @corona10, @shihai1991
PRs	bpo-42035: Add a PyType_GetName() to get type's short name. #23903 bpo-42035: Add a PyType_GetQualName() to get type's qualified name. #27551 bpo-42035: Enhance test_get_type_name() of _testcapi. #27649

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-08-17.15:33:17.695>
created_at = <Date 2020-10-14.13:52:05.070>
labels = ['expert-C-API', 'type-feature']
title = '[C API] PyType_GetSlot cannot get tp_name'
updated_at = <Date 2021-08-17.15:33:17.694>
user = 'https://bugs.python.org/fancitron'

bugs.python.org fields:

activity = <Date 2021-08-17.15:33:17.694>
actor = 'petr.viktorin'
assignee = 'none'
closed = True
closed_date = <Date 2021-08-17.15:33:17.695>
closer = 'petr.viktorin'
components = ['C API']
creation = <Date 2020-10-14.13:52:05.070>
creator = 'fancitron'
dependencies = []
files = []
hgrepos = []
issue_num = 42035
keywords = ['patch']
message_count = 22.0
messages = ['378616', '379102', '379109', '379291', '379416', '383070', '385274', '385281', '385282', '385390', '385433', '386080', '398292', '398311', '398466', '398468', '398766', '399261', '399749', '399755', '399757', '399760']
nosy_count = 5.0
nosy_names = ['vstinner', 'petr.viktorin', 'corona10', 'shihai1991', 'fancitron']
pr_nums = ['23903', '27551', '27649']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue42035'
versions = []

[fancitron]

In the Limited API (where PyTypeObject is opaque), there is no way to retrieve the tp_name of a type object. The PyType_GetSlot() function doesn’t define a slot ID Py_tp_name. This makes it inconvenient to port existing code to the Limited API. Is this intentional? How to work around this?

[encukou]

The slots are originally intended for defining types (PyType_FromSpec); PyType_GetSlot is not as useful as it could be.
tp_name can be exposed, but it needs to also be handled properly PyType_FromSpec -- e.g. raise an error.

[encukou]

Ah, scratch that: PyType_GetSlot returns a function pointer.
To be correct, we should to expose a new function like PyType_GetName.

It's true that CPython currently doesn't always honor the distinction between data and function pointers, but the C standard says they're distinct, so it might bite us in the future.

[fancitron]

True enough. Btw, PyType_FromSpec accepts Py_tp_doc (char *), Py_tp_base (PyTypeObject *), etc ... so to be strictly standard compliant, a union would be necessary.

PyType_GetName() sounds great.

One "proper" workaround at the moment is PyObject_GetAttrString(Py_TYPE(x), "__name__") and then process the result. This is somewhat "heavy" and strips the module name. "Py_tp_name" provides a convenient, exception safe, and backward compatible way to access tp_name.

What I actually do right now is to access the (opaque) PyTypeObject::tp_name by pointer offset. This certain defies the purpose of stable ABI!

[encukou]

Yes, for some cases the ship has sailed. I don't think we can add unions now -- the stable ABI needs to be stable.

[encukou]

I'll be happy to review adding a PyType_GetName function.

[encukou]

Now that I see the implementation (and now that I'm spending a lot of time trying to formalize what is good stable API), I see a problem with PyType_GetName: it effectively returns a borrowed reference.
The proposed docs say:

Callers can hold [the retuned] pointer until the type has been deallocated.

This is not friendly to alternate Python implementations. For example, if tp_name is stored as UTF-32, it would need to generate the char* data -- and then retain it until the class is deallocated.
I guess the "correct" way would be to return a Py_buffer, which would (in CPython) reference the class.

Victor, what do you think?

[vstinner]

New C API functions must not return borrowed references, but strong references.

[vstinner]

A type has different names:

• short name: "name"
• qualified name: "module.name"

Which one should be returned by PyType_GetName()? Is there a warranty that it's always short or always qualified?