Add _StrPromise as a special type for Promise objects for str.

This allows the user to access methods defined on lazy strings while
still letting mypy be aware of that they are not instances of `str`.

The definitions for some of the magic methods are pulled from typeshed. We need
those definitions in the stubs so that `_StrPromise` objects will work properly
with operators, as refining operator types is tricky with the mypy
plugins API.

The rest of the methods will be covered by an attribute hook.

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

django-stubs/utils/functional.pyi

@@ -1,8 +1,9 @@
 import sys
 from functools import wraps as wraps  # noqa: F401
-from typing import Any, Callable, Generic, List, Optional, Tuple, Type, TypeVar, Union, overload
+from typing import Any, Callable, Generic, List, Optional, Sequence, Tuple, Type, TypeVar, Union, overload
 
 from django.db.models.base import Model
+from typing_extensions import SupportsIndex
 
 if sys.version_info < (3, 8):
     from typing_extensions import Protocol
@@ -31,10 +32,24 @@ class Promise:
     def __radd__(self, other: Any) -> Any: ...
     def __deepcopy__(self, memo: Any): ...
 
+class _StrPromise(Promise, Sequence[str]):
+    def __add__(self, __s: str) -> str: ...
+    # Incompatible with Sequence.__contains__
+    def __contains__(self, __o: str) -> bool: ...  # type: ignore[override]
+    def __ge__(self, __x: str) -> bool: ...
+    def __getitem__(self, __i: SupportsIndex | slice) -> str: ...
+    def __gt__(self, __x: str) -> bool: ...
+    def __le__(self, __x: str) -> bool: ...
+    def __lt__(self, __x: str) -> bool: ...
+    def __mod__(self, __x: Any) -> str: ...
+    def __mul__(self, __n: SupportsIndex) -> str: ...
+    def __rmul__(self, __n: SupportsIndex) -> str: ...
+    def __getnewargs__(self) -> tuple[str]: ...
+
 _C = TypeVar("_C", bound=Callable)
 
 def lazy(func: _C, *resultclasses: Any) -> _C: ...
-def lazystr(text: Any) -> str: ...
+def lazystr(text: Any) -> _StrPromise: ...
 def keep_lazy(*resultclasses: Any) -> Callable: ...
 def keep_lazy_text(func: Callable) -> Callable: ...
 

django-stubs/utils/translation/__init__.pyi

@@ -4,7 +4,7 @@ from contextlib import ContextDecorator
 from typing import Any, Callable, Optional, Type, Union
 
 from django.http.request import HttpRequest
-from django.utils.functional import Promise
+from django.utils.functional import _StrPromise
 
 LANGUAGE_SESSION_KEY: str
 
@@ -33,10 +33,10 @@ def pgettext(context: str, message: str) -> str: ...
 def npgettext(context: str, singular: str, plural: str, number: int) -> str: ...
 
 # lazy evaluated translation functions
-def gettext_lazy(message: str) -> Promise: ...
-def pgettext_lazy(context: str, message: str) -> Promise: ...
-def ngettext_lazy(singular: str, plural: str, number: Union[int, str, None] = ...) -> Promise: ...
-def npgettext_lazy(context: str, singular: str, plural: str, number: Union[int, str, None] = ...) -> Promise: ...
+def gettext_lazy(message: str) -> _StrPromise: ...
+def pgettext_lazy(context: str, message: str) -> _StrPromise: ...
+def ngettext_lazy(singular: str, plural: str, number: Union[int, str, None] = ...) -> _StrPromise: ...
+def npgettext_lazy(context: str, singular: str, plural: str, number: Union[int, str, None] = ...) -> _StrPromise: ...
 
 # NOTE: These translation functions are deprecated and removed in Django 4.0. We should remove them when we drop
 # support for 3.2