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gh-122634: Deprecate __class_getitem__ on metaclasses #122743

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gh-122634: Deprecate `__class_getitem__` on metaclasses
sobolevn Aug 6, 2024
22c7287
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sobolevn Aug 14, 2024
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7 changes: 7 additions & 0 deletions Doc/whatsnew/3.14.rst
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Expand Up @@ -189,6 +189,13 @@ asyncio
Deprecated
==========

* Currently, subscripting a class object with a metaclass
that defines the :meth:`~object.__class_getitem__` method
would invoke the metaclass's method. This behavior is now deprecated
and will be removed in Python 3.16.
To make instances of a metaclass subscriptable,
define a :meth:`~object.__getitem__` method on the metaclass.

* :mod:`builtins`:
Passing a complex number as the *real* or *imag* argument in the
:func:`complex` constructor is now deprecated; it should only be passed
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31 changes: 31 additions & 0 deletions Lib/test/test_class.py
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Expand Up @@ -787,6 +787,37 @@ def __init__(self, obj):
Type(i)
self.assertEqual(calls, 100)

def testClassGetItemIsDeprecatedOnMetaclasses(self):
import warnings

class Meta(type):
def __class_getitem__(self, arg):
return 1
class NoMethod(metaclass=Meta): ...
class HasOwnMethod(metaclass=Meta):
def __class_getitem__(self, arg):
return 2

with self.assertWarnsRegex(
DeprecationWarning,
"Accessing __class_getitem__ on a metaclass",
):
res = NoMethod[int]
self.assertEqual(res, 1)

with warnings.catch_warnings(record=True) as w:
self.assertEqual(HasOwnMethod[int], 2)
self.assertEqual(len(w), 0)

class CorrectMeta(type):
def __getitem__(self, arg):
return 3
class NoClassMethod(metaclass=CorrectMeta): ...

with warnings.catch_warnings(record=True) as w:
self.assertEqual(NoClassMethod[int], 3)
self.assertEqual(len(w), 0)


from _testinternalcapi import has_inline_values

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6 changes: 6 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2024-08-06-20-20-56.gh-issue-122634.BUON7A.rst
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@@ -0,0 +1,6 @@
Currently, subscripting a class object with a metaclass
that defines the :meth:`~object.__class_getitem__` method
would invoke the metaclass's method. This behavior is now deprecated
and will be removed in Python 3.16.
To make instances of a metaclass subscriptable,
define a :meth:`~object.__getitem__` method on the metaclass.
10 changes: 10 additions & 0 deletions Objects/typeobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -5616,6 +5616,16 @@ _Py_type_getattro_impl(PyTypeObject *type, PyObject *name, int * suppress_missin
res = meta_get(meta_attribute, (PyObject *)type,
(PyObject *)metatype);
Py_DECREF(meta_attribute);
if (res && PyUnicode_EqualToUTF8(name, "__class_getitem__")) {
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This makes it so the deprecation gets triggered if you use regular attribute access to access the attribute, but I don't think that's right:

>>> class M(type):
...     def __class_getitem__(*args):
...         return "CGI"
...         
>>> class A(metaclass=M): pass
... 
>>> A.__class_getitem__
<python-input-7>:1: DeprecationWarning: Accessing __class_getitem__ on a metaclass is deprecated since 3.14 and will be removed in 3.16. Instead, define a regular __getitem__ method on a metaclass, if you need this behavior.
  A.__class_getitem__
<bound method M.__class_getitem__ of <class '__main__.M'>>

Instead, the deprecation should trigger only if the lookup is triggered by the subscript operator, like in this example:

>>> A["x"]
<python-input-8>:1: DeprecationWarning: Accessing __class_getitem__ on a metaclass is deprecated since 3.14 and will be removed in 3.16. Instead, define a regular __getitem__ method on a metaclass, if you need this behavior.
  A["x"]
'CGI'

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I don't think that there's any clean way to do that. We don't have any info about that in _Py_type_getattro_impl (nor in its public version).

Passing this info would require either a global state or a new parameter in several functions (some of which are public).

Maybe I am missing something?

I don't think that .__class_getitem__ access should really bother us, because:

  1. Users should not access magic method directly
  2. Warning is valid in some cases

if (PyErr_WarnEx(PyExc_DeprecationWarning,
"Accessing __class_getitem__ on a metaclass is deprecated "
"since 3.14 and will be removed in 3.16. "
"Instead, define a regular __getitem__ method on a metaclass, "
"if you need this behavior.",
1) < 0) {
return NULL;
}
}
return res;
}

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