Narrow RegexValidator.regex instance level type to Pattern[str]

django-stubs/core/validators.pyi

@@ -1,7 +1,7 @@
 from collections.abc import Callable, Collection, Sequence, Sized
 from decimal import Decimal
 from re import Pattern, RegexFlag
-from typing import Any, Generic, TypeAlias, TypeVar, overload, type_check_only
+from typing import Any, TypeAlias
 
 from django.core.files.base import File
 from django.utils.deconstruct import _Deconstructible
@@ -13,19 +13,8 @@ _Regex: TypeAlias = str | Pattern[str]
 
 _ValidatorCallable: TypeAlias = Callable[[Any], None]  # noqa: PYI047
 
-_ClassT = TypeVar("_ClassT")
-_InstanceT = TypeVar("_InstanceT")
-
-@type_check_only
-class _ClassOrInstanceAttribute(Generic[_ClassT, _InstanceT]):
-    @overload
-    def __get__(self, instance: None, owner: type[object]) -> _ClassT: ...
-    @overload
-    def __get__(self, instance: object, owner: type[object]) -> _InstanceT: ...
-    def __set__(self, instance: object, value: _InstanceT) -> None: ...
-
 class RegexValidator(_Deconstructible):
-    regex: _ClassOrInstanceAttribute[_Regex, Pattern[str]]
+    regex: Pattern[str]
     message: _StrOrPromise
     code: str
     inverse_match: bool

scripts/stubtest/allowlist.txt

@@ -551,8 +551,9 @@ django.contrib.gis.forms.BaseModelFormSet.save_m2m
 # Dynamically generated in https://github.com/django/django/blob/0ee06c04e0256094270db3ffe8b5dafa6a8457a3/django/core/mail/backends/locmem.py#L24
 django.core.mail.outbox
 
-# We use a trick using a descriptor class to represent an attribute with different type at the class and instance level.
-# Here to narrow `str | Pattern[str]` at the class level to `Pattern[str]` at the instance level.
+# The type system does not allow to represent an attribute with different type at the class level (`str | Pattern[str]`)
+# and instance (`Pattern[str]`) level. But in order to have more precise type at the instance level, we restrict the types
+# allowed at the class level to a subset. In an ideal world, these should probably have different attribute names.
 django.core.validators.RegexValidator.regex
 django.contrib.auth.validators.ASCIIUsernameValidator.regex
 django.contrib.auth.validators.UnicodeUsernameValidator.regex

tests/assert_type/core/test_validators.py

@@ -5,18 +5,28 @@
 from django.core.validators import RegexValidator
 from typing_extensions import assert_type
 
-assert_type(RegexValidator.regex, str | Pattern[str])
 assert_type(RegexValidator().regex, Pattern[str])
 RegexValidator().regex = re.compile("")
 
-assert_type(UnicodeUsernameValidator.regex, str | Pattern[str])
 assert_type(UnicodeUsernameValidator().regex, Pattern[str])
 UnicodeUsernameValidator().regex = re.compile("")
 
 # expect "Pattern[str]"
 RegexValidator().regex = ""  # type: ignore[assignment] # pyright: ignore[reportAttributeAccessIssue]
 UnicodeUsernameValidator().regex = ""  # type: ignore[assignment] # pyright: ignore[reportAttributeAccessIssue]
 
-# expect "_ClassOrInstanceAttribute[Union[str, Pattern[str]], Pattern[str]]"
-RegexValidator.regex = "anything fails here"  # type: ignore[assignment] # pyright: ignore[reportAttributeAccessIssue]
-UnicodeUsernameValidator.regex = "anything fails here"  # type: ignore[assignment] # pyright: ignore[reportAttributeAccessIssue]
+
+class RegexSubtype(RegexValidator):
+    regex = re.compile("abc")
+
+
+# We would like to improve on these, it should allow "str | Pattern[str]":
+assert_type(RegexValidator.regex, Pattern[str])
+assert_type(UnicodeUsernameValidator.regex, Pattern[str])
+
+RegexValidator.regex = ""  # type: ignore[assignment] # pyright: ignore[reportAttributeAccessIssue]
+UnicodeUsernameValidator.regex = ""  # type: ignore[assignment] # pyright: ignore[reportAttributeAccessIssue]
+
+
+class StrSubtype(RegexValidator):
+    regex = "abc"  # type: ignore[assignment] # pyright: ignore[reportAttributeAccessIssue]