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

django-stubs/core/validators.pyi

@@ -14,7 +14,7 @@ _Regex: TypeAlias = str | Pattern[str]
 _ValidatorCallable: TypeAlias = Callable[[Any], None]  # noqa: PYI047
 
 class RegexValidator(_Deconstructible):
-    regex: _Regex  # Pattern[str] on instance, but may be str on class definition
+    regex: Pattern[str]
     message: _StrOrPromise
     code: str
     inverse_match: bool

scripts/stubtest/allowlist.txt

@@ -550,3 +550,10 @@ 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
+
+# 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

@@ -0,0 +1,32 @@
+import re
+from re import Pattern
+
+from django.contrib.auth.validators import UnicodeUsernameValidator
+from django.core.validators import RegexValidator
+from typing_extensions import assert_type
+
+assert_type(RegexValidator().regex, Pattern[str])
+RegexValidator().regex = re.compile("")
+
+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]
+
+
+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[reportAssignmentType]