typing: Populate POST and _files with `cast` and `setattr`.

`POST` is an immutable attribute and `_files` is an internal attribute
of `HttpRequest`. With type annotations provided by `django-stubs`, mypy
stops us from modifying these attributes. This uses `cast` and `setattr`
to avoid typing issues.

This is a part of django-stubs refactorings.

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

zerver/decorator.py

@@ -4,7 +4,18 @@ import logging
 import urllib
 from functools import wraps
 from io import BytesIO
-from typing import Callable, Dict, Optional, Sequence, Set, TypeVar, Union, cast, overload
+from typing import (
+    TYPE_CHECKING,
+    Callable,
+    Dict,
+    Optional,
+    Sequence,
+    Set,
+    TypeVar,
+    Union,
+    cast,
+    overload,
+)
 
 import django_otp
 import orjson
@@ -62,6 +73,9 @@ if settings.ZILENCER_ENABLED:
         get_remote_server_by_uuid,
     )
 
+if TYPE_CHECKING:
+    from django.http.request import _ImmutableQueryDict
+
 rate_limiter_logger = logging.getLogger("zerver.lib.rate_limiter")
 
 webhook_logger = logging.getLogger("zulip.zerver.webhooks")
@@ -719,15 +733,26 @@ def process_as_post(view_func: ViewFuncT) -> ViewFuncT:
         if not request.POST:
             # Only take action if POST is empty.
             if request.content_type == "multipart/form-data":
-                # Note that request._files is just the private attribute that backs the
-                # FILES property, so we are essentially setting request.FILES here.  (In
-                # Django 1.5 FILES was still a read-only property.)
-                request.POST, request._files = MultiPartParser(
+                POST, _files = MultiPartParser(
                     request.META,
                     BytesIO(request.body),
                     request.upload_handlers,
                     request.encoding,
                 ).parse()
+                # request.POST is an immutable QueryDict in most cases, while
+                # MultiPartParser.parse() returns a mutable instance of QueryDict.
+                # This can be fix when https://code.djangoproject.com/ticket/17235
+                # is resolved.
+                # django-stubs makes QueryDict of different mutabilities incompatible
+                # types. There is no way to acknowledge the django-stubs mypy plugin
+                # the change of POST's mutability, so we bypass the check with cast.
+                # See also: https://github.com/typeddjango/django-stubs/pull/925#issue-1206399444
+                POST._mutable = False
+                request.POST = cast("_ImmutableQueryDict", POST)
+                # Note that request._files is just the private attribute that backs the
+                # FILES property, so we are essentially setting request.FILES here.  (In
+                # Django 3.2 FILES was still a read-only property.)
+                setattr(request, "_files", _files)
             elif request.content_type == "application/x-www-form-urlencoded":
                 request.POST = QueryDict(request.body, encoding=request.encoding)